diff --git a/packages/browser-integration-tests/fixtures/loader.js b/packages/browser-integration-tests/fixtures/loader.js
index c95b5eabdcac..36d7d2401856 100644
--- a/packages/browser-integration-tests/fixtures/loader.js
+++ b/packages/browser-integration-tests/fixtures/loader.js
@@ -1,4 +1,5 @@
-!function(n,e,t,r,o,a,i,c,_,p){for(var s=p,forceLoad=!1,f=0;f<document.scripts.length;f++)if(document.scripts[f].src.indexOf(i)>-1){s&&"no"===document.scripts[f].getAttribute("data-lazy")&&(s=!1);break}var u=!1,l=[],d=function(n){("e"in n||"p"in n||n.f&&n.f.indexOf("capture")>-1||n.f&&n.f.indexOf("showReportDialog")>-1)&&s&&E(l),d.data.push(n)};function E(i){if(!u){u=!0;var p=e.scripts[0],s=e.createElement(t);s.src=c,s.crossOrigin="anonymous",s.addEventListener("load",(function(){try{n[r]&&n[r].__SENTRY_LOADER__&&(n[r]=R),n[o]&&n[o].__SENTRY_LOADER__&&(n[o]=h),n.SENTRY_SDK_SOURCE="loader";var e=n[a],t=e.init,c=[];_.tracesSampleRate&&c.push(new e.BrowserTracing),(_.replaysSessionSampleRate||_.replaysOnErrorSampleRate)&&c.push(new e.Replay),c.length&&(_.integrations=c),e.init=function(n){var e=_;for(var r in n)Object.prototype.hasOwnProperty.call(n,r)&&(e[r]=n[r]);t(e)},function(e,t){try{for(var a=0;a<e.length;a++)"function"==typeof e[a]&&e[a]();var i=d.data,c=!(void 0===(u=n.__SENTRY__)||!u.hub||!u.hub.getClient());i.sort((function(n){return"init"===n.f?-1:0}));var _=!1;for(a=0;a<i.length;a++)if(i[a].f){_=!0;var p=i[a];!1===c&&"init"!==p.f&&t.init(),c=!0,t[p.f].apply(t,p.a)}!1===c&&!1===_&&t.init();var s=n[r],f=n[o];for(a=0;a<i.length;a++)"e"in i[a]&&s?s.apply(n,i[a].e):"p"in i[a]&&f&&f.apply(n,[i[a].p])}catch(n){console.error(n)}var u}(i,e)}catch(n){console.error(n)}})),p.parentNode.insertBefore(s,p)}}d.data=[],n[a]=n[a]||{},n[a].onLoad=function(n){l.push(n),s&&!forceLoad||E(l)},n[a].forceLoad=function(){forceLoad=!0,s&&setTimeout((function(){E(l)}))},["init","addBreadcrumb","captureMessage","captureException","captureEvent","configureScope","withScope","showReportDialog"].forEach((function(e){n[a][e]=function(){d({f:e,a:arguments})}}));var R=n[r];n[r]=function(){d({e:[].slice.call(arguments)}),R&&R.apply(n,arguments)},n[r].__SENTRY_LOADER__=!0;var h=n[o];n[o]=function(e){d({p:"reason"in e?e.reason:"detail"in e&&"reason"in e.detail?e.detail.reason:e}),h&&h.apply(n,arguments)},n[o].__SENTRY_LOADER__=!0,s||setTimeout((function(){E(l)}))}(
+!function(n,e,r,t,a,i,o,c,_,f){for(var p=f,forceLoad=!1,s=0;s<document.scripts.length;s++)if(document.scripts[s].src.indexOf(o)>-1){p&&"no"===document.scripts[s].getAttribute("data-lazy")&&(p=!1);break}var u=!1,l=[],d=function(n){("e"in n||"p"in n||n.f&&n.f.indexOf("capture")>-1||n.f&&n.f.indexOf("showReportDialog")>-1)&&p&&R(l),d.data.push(n)};function R(o){if(!u){u=!0;var f=e.scripts[0],p=e.createElement(r);p.src=c,p.crossOrigin="anonymous",p.addEventListener("load",(function(){try{n[t]&&n[t].__SENTRY_LOADER__&&(n[t]=E),n[a]&&n[a].__SENTRY_LOADER__&&(n[a]=v),n.SENTRY_SDK_SOURCE="loader";var e=n[i],r=e.init;e.init=function(n){var t=_;for(var a in n)Object.prototype.hasOwnProperty.call(n,a)&&(t[a]=n[a]);!function(n,e){var r=n.integrations||[];if(!Array.isArray(r))return;var t=r.map((function(n){return n.name}));n.tracesSampleRate&&-1===t.indexOf("BrowserTracing")&&r.push(new e.BrowserTracing);(n.replaysSessionSampleRate||n.replaysOnErrorSampleRate)&&-1===t.indexOf("Replay")&&r.push(new e.Replay);n.integrations=r}(t,e),r(t)},function(e,r){try{for(var i=0;i<e.length;i++)"function"==typeof e[i]&&e[i]();var o=d.data,c=!(void 0===(u=n.__SENTRY__)||!u.hub||!u.hub.getClient());o.sort((function(n){return"init"===n.f?-1:0}));var _=!1;for(i=0;i<o.length;i++)if(o[i].f){_=!0;var f=o[i];!1===c&&"init"!==f.f&&r.init(),c=!0,r[f.f].apply(r,f.a)}!1===c&&!1===_&&r.init();var p=n[t],s=n[a];for(i=0;i<o.length;i++)"e"in o[i]&&p?p.apply(n,o[i].e):"p"in o[i]&&s&&s.apply(n,[o[i].p])}catch(n){console.error(n)}var u}(o,e)}catch(n){console.error(n)}})),f.parentNode.insertBefore(p,f)}}d.data=[],n[i]=n[i]||{},n[i].onLoad=function(n){l.push(n),p&&!forceLoad||R(l)},n[i].forceLoad=function(){forceLoad=!0,p&&setTimeout((function(){R(l)}))},["init","addBreadcrumb","captureMessage","captureException","captureEvent","configureScope","withScope","showReportDialog"].forEach((function(e){n[i][e]=function(){d({f:e,a:arguments})}}));var E=n[t];n[t]=function(){d({e:[].slice.call(arguments)}),E&&E.apply(n,arguments)},n[t].__SENTRY_LOADER__=!0;var v=n[a];n[a]=function(e){d({p:"reason"in e?e.reason:"detail"in e&&"reason"in e.detail?e.detail.reason:e}),v&&v.apply(n,arguments)},n[a].__SENTRY_LOADER__=!0,p||setTimeout((function(){R(l)}))}
+(
   window,
   document,
   'script',
diff --git a/packages/browser-integration-tests/loader-suites/loader/onLoad/customBrowserTracing/init.js b/packages/browser-integration-tests/loader-suites/loader/onLoad/customBrowserTracing/init.js
new file mode 100644
index 000000000000..cf30bc698e53
--- /dev/null
+++ b/packages/browser-integration-tests/loader-suites/loader/onLoad/customBrowserTracing/init.js
@@ -0,0 +1,15 @@
+import * as Sentry from '@sentry/browser';
+
+window.Sentry = Sentry;
+window._testBaseTimestamp = performance.timeOrigin / 1000;
+
+Sentry.onLoad(function () {
+  Sentry.init({
+    integrations: [
+      // Without this syntax, this will be re-written by the test framework
+      new window['Sentry'].BrowserTracing({
+        tracePropagationTargets: ['http://localhost:1234'],
+      }),
+    ],
+  });
+});
diff --git a/packages/browser-integration-tests/loader-suites/loader/onLoad/customBrowserTracing/test.ts b/packages/browser-integration-tests/loader-suites/loader/onLoad/customBrowserTracing/test.ts
new file mode 100644
index 000000000000..19ae8be8a366
--- /dev/null
+++ b/packages/browser-integration-tests/loader-suites/loader/onLoad/customBrowserTracing/test.ts
@@ -0,0 +1,33 @@
+import { expect } from '@playwright/test';
+
+import { sentryTest } from '../../../../utils/fixtures';
+import { envelopeRequestParser, shouldSkipTracingTest, waitForTransactionRequest } from '../../../../utils/helpers';
+
+sentryTest('should handle custom added BrowserTracing integration', async ({ getLocalTestUrl, page }) => {
+  if (shouldSkipTracingTest()) {
+    sentryTest.skip();
+  }
+
+  const req = waitForTransactionRequest(page);
+
+  const url = await getLocalTestUrl({ testDir: __dirname });
+  await page.goto(url);
+
+  const eventData = envelopeRequestParser(await req);
+  const timeOrigin = await page.evaluate<number>('window._testBaseTimestamp');
+
+  const { start_timestamp: startTimestamp } = eventData;
+
+  expect(startTimestamp).toBeCloseTo(timeOrigin, 1);
+
+  expect(eventData.contexts?.trace?.op).toBe('pageload');
+  expect(eventData.spans?.length).toBeGreaterThan(0);
+  expect(eventData.transaction_info?.source).toEqual('url');
+
+  const tracePropagationTargets = await page.evaluate(() => {
+    const browserTracing = (window as any).Sentry.getCurrentHub().getClient().getIntegrationById('BrowserTracing');
+    return browserTracing.options.tracePropagationTargets;
+  });
+
+  expect(tracePropagationTargets).toEqual(['http://localhost:1234']);
+});
diff --git a/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrations/init.js b/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrations/init.js
new file mode 100644
index 000000000000..a5440c1979c5
--- /dev/null
+++ b/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrations/init.js
@@ -0,0 +1,19 @@
+import * as Sentry from '@sentry/browser';
+
+window.Sentry = Sentry;
+
+class CustomIntegration {
+  constructor() {
+    this.name = 'CustomIntegration';
+  }
+
+  setupOnce() {}
+}
+
+Sentry.onLoad(function () {
+  Sentry.init({
+    integrations: [new CustomIntegration()],
+  });
+
+  window.__sentryLoaded = true;
+});
diff --git a/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrations/subject.js b/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrations/subject.js
new file mode 100644
index 000000000000..707e54eefe7a
--- /dev/null
+++ b/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrations/subject.js
@@ -0,0 +1 @@
+Sentry.forceLoad();
diff --git a/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrations/test.ts b/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrations/test.ts
new file mode 100644
index 000000000000..836d10a8d486
--- /dev/null
+++ b/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrations/test.ts
@@ -0,0 +1,40 @@
+import { expect } from '@playwright/test';
+
+import { sentryTest } from '../../../../utils/fixtures';
+import { shouldSkipTracingTest } from '../../../../utils/helpers';
+import { shouldSkipReplayTest } from '../../../../utils/replayHelpers';
+
+sentryTest('should handle custom added integrations & default integrations', async ({ getLocalTestUrl, page }) => {
+  const shouldHaveReplay = !shouldSkipReplayTest();
+  const shouldHaveBrowserTracing = !shouldSkipTracingTest();
+
+  await page.route('https://dsn.ingest.sentry.io/**/*', route => {
+    return route.fulfill({
+      status: 200,
+      contentType: 'application/json',
+      body: JSON.stringify({ id: 'test-id' }),
+    });
+  });
+
+  const url = await getLocalTestUrl({ testDir: __dirname });
+  await page.goto(url);
+
+  await page.waitForFunction(() => {
+    return (window as any).__sentryLoaded;
+  });
+
+  const hasCustomIntegration = await page.evaluate(() => {
+    return !!(window as any).Sentry.getCurrentHub().getClient().getIntegrationById('CustomIntegration');
+  });
+
+  const hasReplay = await page.evaluate(() => {
+    return !!(window as any).Sentry.getCurrentHub().getClient().getIntegrationById('Replay');
+  });
+  const hasBrowserTracing = await page.evaluate(() => {
+    return !!(window as any).Sentry.getCurrentHub().getClient().getIntegrationById('BrowserTracing');
+  });
+
+  expect(hasCustomIntegration).toEqual(true);
+  expect(hasReplay).toEqual(shouldHaveReplay);
+  expect(hasBrowserTracing).toEqual(shouldHaveBrowserTracing);
+});
diff --git a/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrationsFunction/init.js b/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrationsFunction/init.js
new file mode 100644
index 000000000000..4c1e600794d5
--- /dev/null
+++ b/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrationsFunction/init.js
@@ -0,0 +1,19 @@
+import * as Sentry from '@sentry/browser';
+
+window.Sentry = Sentry;
+
+class CustomIntegration {
+  constructor() {
+    this.name = 'CustomIntegration';
+  }
+
+  setupOnce() {}
+}
+
+Sentry.onLoad(function () {
+  Sentry.init({
+    integrations: integrations => [new CustomIntegration()].concat(integrations),
+  });
+
+  window.__sentryLoaded = true;
+});
diff --git a/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrationsFunction/subject.js b/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrationsFunction/subject.js
new file mode 100644
index 000000000000..707e54eefe7a
--- /dev/null
+++ b/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrationsFunction/subject.js
@@ -0,0 +1 @@
+Sentry.forceLoad();
diff --git a/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrationsFunction/test.ts b/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrationsFunction/test.ts
new file mode 100644
index 000000000000..7ff989922f33
--- /dev/null
+++ b/packages/browser-integration-tests/loader-suites/loader/onLoad/customIntegrationsFunction/test.ts
@@ -0,0 +1,38 @@
+import { expect } from '@playwright/test';
+
+import { sentryTest } from '../../../../utils/fixtures';
+
+sentryTest(
+  'should not add default integrations if integrations function is provided',
+  async ({ getLocalTestUrl, page }) => {
+    await page.route('https://dsn.ingest.sentry.io/**/*', route => {
+      return route.fulfill({
+        status: 200,
+        contentType: 'application/json',
+        body: JSON.stringify({ id: 'test-id' }),
+      });
+    });
+
+    const url = await getLocalTestUrl({ testDir: __dirname });
+    await page.goto(url);
+
+    await page.waitForFunction(() => {
+      return (window as any).__sentryLoaded;
+    });
+
+    const hasCustomIntegration = await page.evaluate(() => {
+      return !!(window as any).Sentry.getCurrentHub().getClient().getIntegrationById('CustomIntegration');
+    });
+
+    const hasReplay = await page.evaluate(() => {
+      return !!(window as any).Sentry.getCurrentHub().getClient().getIntegrationById('Replay');
+    });
+    const hasBrowserTracing = await page.evaluate(() => {
+      return !!(window as any).Sentry.getCurrentHub().getClient().getIntegrationById('BrowserTracing');
+    });
+
+    expect(hasCustomIntegration).toEqual(true);
+    expect(hasReplay).toEqual(false);
+    expect(hasBrowserTracing).toEqual(false);
+  },
+);
diff --git a/packages/browser-integration-tests/loader-suites/loader/onLoad/customReplay/init.js b/packages/browser-integration-tests/loader-suites/loader/onLoad/customReplay/init.js
new file mode 100644
index 000000000000..64d2463ed668
--- /dev/null
+++ b/packages/browser-integration-tests/loader-suites/loader/onLoad/customReplay/init.js
@@ -0,0 +1,14 @@
+import * as Sentry from '@sentry/browser';
+
+window.Sentry = Sentry;
+
+Sentry.onLoad(function () {
+  Sentry.init({
+    integrations: [
+      // Without this syntax, this will be re-written by the test framework
+      new window['Sentry'].Replay({
+        useCompression: false,
+      }),
+    ],
+  });
+});
diff --git a/packages/browser-integration-tests/loader-suites/loader/onLoad/customReplay/test.ts b/packages/browser-integration-tests/loader-suites/loader/onLoad/customReplay/test.ts
new file mode 100644
index 000000000000..f42868cf9ead
--- /dev/null
+++ b/packages/browser-integration-tests/loader-suites/loader/onLoad/customReplay/test.ts
@@ -0,0 +1,35 @@
+import { expect } from '@playwright/test';
+
+import { sentryTest } from '../../../../utils/fixtures';
+import { getReplayEvent, shouldSkipReplayTest, waitForReplayRequest } from '../../../../utils/replayHelpers';
+
+sentryTest('should handle custom added Replay integration', async ({ getLocalTestUrl, page }) => {
+  if (shouldSkipReplayTest()) {
+    sentryTest.skip();
+  }
+
+  await page.route('https://dsn.ingest.sentry.io/**/*', route => {
+    return route.fulfill({
+      status: 200,
+      contentType: 'application/json',
+      body: JSON.stringify({ id: 'test-id' }),
+    });
+  });
+
+  const req = waitForReplayRequest(page);
+
+  const url = await getLocalTestUrl({ testDir: __dirname });
+  await page.goto(url);
+
+  const eventData = getReplayEvent(await req);
+
+  expect(eventData).toBeDefined();
+  expect(eventData.segment_id).toBe(0);
+
+  const useCompression = await page.evaluate(() => {
+    const replay = (window as any).Sentry.getCurrentHub().getClient().getIntegrationById('Replay');
+    return replay._replay.getOptions().useCompression;
+  });
+
+  expect(useCompression).toEqual(false);
+});
diff --git a/packages/browser-integration-tests/package.json b/packages/browser-integration-tests/package.json
index a61de94fb0bc..c8b9c92d19e8 100644
--- a/packages/browser-integration-tests/package.json
+++ b/packages/browser-integration-tests/package.json
@@ -41,7 +41,8 @@
     "test:loader:full": "PW_BUNDLE=loader_tracing_replay yarn test:loader",
     "test:ci": "playwright test ./suites --browser='all' --reporter='line'",
     "test:update-snapshots": "yarn test --update-snapshots --browser='all' && yarn test --update-snapshots",
-    "test:detect-flaky": "ts-node scripts/detectFlakyTests.ts"
+    "test:detect-flaky": "ts-node scripts/detectFlakyTests.ts",
+    "validate:es5": "es-check es5 'fixtures/loader.js'"
   },
   "dependencies": {
     "@babel/preset-typescript": "^7.16.7",
diff --git a/packages/integrations/package.json b/packages/integrations/package.json
index 6b79350fa5a9..6e799214516a 100644
--- a/packages/integrations/package.json
+++ b/packages/integrations/package.json
@@ -44,6 +44,7 @@
     "lint": "run-s lint:prettier lint:eslint",
     "lint:eslint": "eslint . --format stylish",
     "lint:prettier": "prettier --check \"{src,test,scripts}/**/**.ts\"",
+    "validate:es5": "es-check es5 'build/bundles/*.es5*.js'",
     "test": "jest",
     "test:watch": "jest --watch",
     "yalc:publish": "ts-node ../../scripts/prepack.ts --bundles && yalc publish ./build/npm --push"