diff --git a/flow-client/src/test/frontend/FlowTests.ts b/flow-client/src/test/frontend/FlowTests.ts
index 8f19e326138..d874a75ef44 100644
--- a/flow-client/src/test/frontend/FlowTests.ts
+++ b/flow-client/src/test/frontend/FlowTests.ts
@@ -14,6 +14,21 @@ import sinon from 'sinon';
 const $wnd = window as any;
 const flowRoot = window.document.body as any;
 
+/**
+ * Stubs window.Vaadin.Flow.ready and whenReady with sentinel values so the
+ * preservation test can verify that constructing a Flow instance does not
+ * overwrite definitions from the inline bootstrap script. The real
+ * implementations live in flow-server's whenReady.js and are not tested here.
+ */
+const READY_SENTINEL = () => {};
+const WHEN_READY_SENTINEL = () => {};
+function stubWhenReady() {
+  $wnd.Vaadin = $wnd.Vaadin || {};
+  $wnd.Vaadin.Flow = $wnd.Vaadin.Flow || {};
+  $wnd.Vaadin.Flow.ready = READY_SENTINEL;
+  $wnd.Vaadin.Flow.whenReady = WHEN_READY_SENTINEL;
+}
+
 const stubVaadinPushSrc = '/src/test/frontend/stubVaadinPush.js';
 let server: MockXhrServer;
 // A `changes` array that adds a div with 'Foo' text to body
@@ -657,6 +672,13 @@ describe('Flow', () => {
     expect($wnd.Vaadin.connectionState.state).to.equal(ConnectionState.RECONNECTING);
   });
 
+  it('should preserve ready and whenReady defined by inline script after construction', () => {
+    stubWhenReady();
+    new Flow({ imports: () => {} });
+    expect($wnd.Vaadin.Flow.ready).to.equal(READY_SENTINEL);
+    expect($wnd.Vaadin.Flow.whenReady).to.equal(WHEN_READY_SENTINEL);
+  });
+
   it('should pre-attach container element on every navigation', async () => {
     stubServerRemoteFunction('foobar-12345');
     mockInitResponse('foobar-12345');
diff --git a/flow-server/src/main/java/com/vaadin/flow/server/BootstrapHandler.java b/flow-server/src/main/java/com/vaadin/flow/server/BootstrapHandler.java
index 3df1c010367..12850e32619 100644
--- a/flow-server/src/main/java/com/vaadin/flow/server/BootstrapHandler.java
+++ b/flow-server/src/main/java/com/vaadin/flow/server/BootstrapHandler.java
@@ -133,6 +133,12 @@ public class BootstrapHandler extends SynchronizedRequestHandler {
             + "/client.nocache.js";
     private static final String BOOTSTRAP_JS = readResource(
             "BootstrapHandler.js");
+    /**
+     * Inlined into the bootstrap {@code <script>}. Must not contain {@code //}
+     * comments — they would swallow the rest of the line if the inline script
+     * is collapsed to one line by HTML processing.
+     */
+    public static final String WHEN_READY_JS = readResource("whenReady.js");
     private static final String CSS_TYPE_ATTRIBUTE_VALUE = "text/css";
 
     private static final String CAPTION = "caption";
@@ -1194,6 +1200,7 @@ private String getBootstrapJS(ObjectNode initialUIDL,
                 result = result.replace("{{GWT_STAT_EVENTS}}", "");
             }
 
+            result = result.replace("{{WHEN_READY}}", WHEN_READY_JS);
             result = result.replace("{{APP_ID}}", context.getAppId());
             result = result.replace("{{CONFIG_JSON}}", appConfigString);
             // {{INITIAL_UIDL}} should be the last replaced so that it may have
diff --git a/flow-server/src/main/java/com/vaadin/flow/server/communication/IndexHtmlRequestHandler.java b/flow-server/src/main/java/com/vaadin/flow/server/communication/IndexHtmlRequestHandler.java
index e33908e7485..35b6ac57e7b 100644
--- a/flow-server/src/main/java/com/vaadin/flow/server/communication/IndexHtmlRequestHandler.java
+++ b/flow-server/src/main/java/com/vaadin/flow/server/communication/IndexHtmlRequestHandler.java
@@ -583,8 +583,10 @@ private void addInitialFlow(ObjectNode initialJson, Document indexDocument,
                 indexDocument.head(), request);
         Element elm = new Element(SCRIPT);
         elm.attr(SCRIPT_INITIAL, "");
-        elm.appendChild(new DataNode("window.Vaadin = window.Vaadin || {};" + //
-                "window.Vaadin.TypeScript= " + initialJson.toString() + ";"));
+        elm.appendChild(new DataNode("window.Vaadin = window.Vaadin || {};"
+                + "window.Vaadin.Flow = window.Vaadin.Flow || {};"
+                + BootstrapHandler.WHEN_READY_JS + "window.Vaadin.TypeScript= "
+                + initialJson.toString() + ";"));
         indexDocument.head().insertChildren(0, elm);
     }
 
diff --git a/flow-server/src/main/resources/com/vaadin/flow/server/BootstrapHandler.js b/flow-server/src/main/resources/com/vaadin/flow/server/BootstrapHandler.js
index e45a36b9250..3705a873a84 100644
--- a/flow-server/src/main/resources/com/vaadin/flow/server/BootstrapHandler.js
+++ b/flow-server/src/main/resources/com/vaadin/flow/server/BootstrapHandler.js
@@ -42,6 +42,10 @@
   window.Vaadin = window.Vaadin || {};
   window.Vaadin.Flow = window.Vaadin.Flow || {};
 
+  if (!window.Vaadin.Flow.whenReady) {
+    {{WHEN_READY}}
+  }
+
   /**
    * Triggers a CSS animation on an element by adding a class, then
    * removes the class when the animation ends.
diff --git a/flow-server/src/main/resources/com/vaadin/flow/server/whenReady.js b/flow-server/src/main/resources/com/vaadin/flow/server/whenReady.js
new file mode 100644
index 00000000000..3d6f7c03ed1
--- /dev/null
+++ b/flow-server/src/main/resources/com/vaadin/flow/server/whenReady.js
@@ -0,0 +1,27 @@
+window.Vaadin.Flow.ready = async function ({ timeout = 30000 } = {}) {
+  const sleep = (ms) => new Promise((r) => setTimeout(r, ms));
+  const deadline = Date.now() + timeout;
+
+  const isIdle = () => {
+    if (document.readyState !== 'complete') return false;
+    if (window.Vaadin.Flow.devServerIsNotLoaded) return false;
+    const clients = window.Vaadin.Flow.clients;
+    if (!clients) return false;
+    const probes = Object.values(clients).filter((c) => typeof c.isActive === 'function');
+    if (probes.length === 0) return false;
+    return probes.every((c) => !c.isActive());
+  };
+
+  while (!isIdle()) {
+    if (Date.now() >= deadline) {
+      throw new Error('Vaadin.Flow.ready timed out after ' + timeout + 'ms');
+    }
+    await sleep(50);
+  }
+};
+
+window.Vaadin.Flow.whenReady = function (callback) {
+  window.Vaadin.Flow.ready()
+    .catch((e) => console.warn(e.message))
+    .then(callback);
+};
diff --git a/flow-server/src/test/java/com/vaadin/flow/server/communication/IndexHtmlRequestHandlerTest.java b/flow-server/src/test/java/com/vaadin/flow/server/communication/IndexHtmlRequestHandlerTest.java
index 86c5dfff8cc..9486081000a 100644
--- a/flow-server/src/test/java/com/vaadin/flow/server/communication/IndexHtmlRequestHandlerTest.java
+++ b/flow-server/src/test/java/com/vaadin/flow/server/communication/IndexHtmlRequestHandlerTest.java
@@ -397,9 +397,14 @@ public void should_not_add_initialUidl_when_not_includeInitialBootstrapUidl()
         Element initialUidlScript = findScript(scripts,
                 INITIAL_UIDL_SEARCH_STRING);
 
-        assertEquals(
-                "window.Vaadin = window.Vaadin || {};window.Vaadin.TypeScript= {};",
-                initialUidlScript.childNode(0).toString());
+        String scriptContent = initialUidlScript.childNode(0).toString();
+        assertTrue(
+                scriptContent.contains("window.Vaadin = window.Vaadin || {};"));
+        assertTrue(scriptContent
+                .contains("window.Vaadin.Flow = window.Vaadin.Flow || {};"));
+        assertTrue(scriptContent.contains("window.Vaadin.Flow.ready = "));
+        assertTrue(scriptContent.contains("window.Vaadin.Flow.whenReady = "));
+        assertTrue(scriptContent.contains("window.Vaadin.TypeScript= {};"));
         assertEquals("", initialUidlScript.attr("initial"));
     }
 
@@ -448,9 +453,14 @@ public void should_not_initialize_UI_and_add_initialUidl_when_invalid_route()
         Elements scripts = document.head().getElementsByTag("script");
         Element initialUidlScript = findScript(scripts,
                 INITIAL_UIDL_SEARCH_STRING);
-        assertEquals(
-                "window.Vaadin = window.Vaadin || {};window.Vaadin.TypeScript= {};",
-                initialUidlScript.childNode(0).toString());
+        String scriptContent = initialUidlScript.childNode(0).toString();
+        assertTrue(
+                scriptContent.contains("window.Vaadin = window.Vaadin || {};"));
+        assertTrue(scriptContent
+                .contains("window.Vaadin.Flow = window.Vaadin.Flow || {};"));
+        assertTrue(scriptContent.contains("window.Vaadin.Flow.ready = "));
+        assertTrue(scriptContent.contains("window.Vaadin.Flow.whenReady = "));
+        assertTrue(scriptContent.contains("window.Vaadin.TypeScript= {};"));
         assertEquals("", initialUidlScript.attr("initial"));
         assertNull(UI.getCurrent());
     }
diff --git a/vaadin-dev-server/src/main/resources/com/vaadin/base/devserver/dev-mode-not-ready.html b/vaadin-dev-server/src/main/resources/com/vaadin/base/devserver/dev-mode-not-ready.html
index 4df7614405b..171e56c3205 100644
--- a/vaadin-dev-server/src/main/resources/com/vaadin/base/devserver/dev-mode-not-ready.html
+++ b/vaadin-dev-server/src/main/resources/com/vaadin/base/devserver/dev-mode-not-ready.html
@@ -49,7 +49,7 @@
     <script type="text/javascript">
       // Set specific flag for TB and make sure that there are no any clients.
       // It allows to work correct waitForVaadin in TB
-      window.Vaadin = { Flow: { devServerIsNotLoaded: true } };
+      window.Vaadin = { Flow: { devServerIsNotLoaded: true, ready: false, whenReady: false } };
 
       const delay = 200;
       const poll = async () => {