diff --git a/httpcore5-testing/src/test/java/org/apache/hc/core5/testing/nio/TLSIntegrationTest.java b/httpcore5-testing/src/test/java/org/apache/hc/core5/testing/nio/TLSIntegrationTest.java
index e84581832..e38f4f7c2 100644
--- a/httpcore5-testing/src/test/java/org/apache/hc/core5/testing/nio/TLSIntegrationTest.java
+++ b/httpcore5-testing/src/test/java/org/apache/hc/core5/testing/nio/TLSIntegrationTest.java
@@ -451,4 +451,52 @@ public boolean upgrade(
 
     }
 
+    @Test
+    void testHttpsProxyWithConnectAndSecondTls() throws Exception {
+        final TlsStrategy targetTlsStrategy = new BasicServerTlsStrategy(
+                SSLTestContexts.createServerSSLContext());
+        server = createServer(targetTlsStrategy);
+        server.start();
+        final ListenerEndpoint targetListener = server
+                .listen(new InetSocketAddress("localhost", 0), URIScheme.HTTPS)
+                .get(TIMEOUT.getDuration(), TIMEOUT.getTimeUnit());
+        final InetSocketAddress targetAddress = (InetSocketAddress) targetListener.getAddress();
+
+        final HttpAsyncServer proxyServer = createServer(
+                new BasicServerTlsStrategy(SSLTestContexts.createServerSSLContext()));
+        proxyServer.start();
+        final ListenerEndpoint proxyListener = proxyServer
+                .listen(new InetSocketAddress("localhost", 0), URIScheme.HTTPS)
+                .get(TIMEOUT.getDuration(), TIMEOUT.getTimeUnit());
+        final InetSocketAddress proxyAddress = (InetSocketAddress) proxyListener.getAddress();
+
+        final TlsStrategy clientTlsStrategy = new BasicClientTlsStrategy(
+                SSLTestContexts.createClientSSLContext());
+        client = createClient(clientTlsStrategy);
+        client.start();
+
+        final HttpHost proxy = new HttpHost("https", "localhost", proxyAddress.getPort());
+        final HttpHost target = new HttpHost("https", "localhost", targetAddress.getPort());
+
+        final Future<Message<HttpResponse, String>> future = client.execute(
+                proxy,
+                new BasicRequestProducer(
+                        Method.POST,
+                        target,
+                        "/test",
+                        new StringAsyncEntityProducer("ping", ContentType.TEXT_PLAIN)
+                ),
+                new BasicResponseConsumer<>(new StringAsyncEntityConsumer()),
+                TIMEOUT, null
+        );
+
+        final Message<HttpResponse, String> response = future
+                .get(TIMEOUT.getDuration(), TIMEOUT.getTimeUnit());
+        Assertions.assertNotNull(response);
+        Assertions.assertEquals(200, response.getHead().getCode());
+        Assertions.assertEquals("ping", response.getBody());
+
+        proxyServer.close(CloseMode.IMMEDIATE);
+    }
+
 }
diff --git a/httpcore5/src/main/java/org/apache/hc/core5/reactor/InternalDataChannel.java b/httpcore5/src/main/java/org/apache/hc/core5/reactor/InternalDataChannel.java
index d290939b4..f2e244fb1 100644
--- a/httpcore5/src/main/java/org/apache/hc/core5/reactor/InternalDataChannel.java
+++ b/httpcore5/src/main/java/org/apache/hc/core5/reactor/InternalDataChannel.java
@@ -226,6 +226,19 @@ public void startTls(
             final SSLSessionVerifier verifier,
             final Timeout handshakeTimeout,
             final FutureCallback<TransportSecurityLayer> callback) {
+
+        // If we already performed TLS (e.g. to the HTTPS proxy), clear out the old session
+        if (tlsSessionRef.get() != null) {
+            // Drop the previous SSLIOSession so we can start fresh
+            tlsSessionRef.set(null);
+            // Revert to raw TCP I/O before installing the new TLS layer
+            currentSessionRef.set(
+                    ioSessionDecorator != null
+                            ? ioSessionDecorator.decorate(ioSession)
+                            : ioSession
+            );
+        }
+
         final SSLIOSession sslioSession = new SSLIOSession(
                 endpoint != null ? endpoint : initialEndpoint,
                 ioSession,
@@ -247,11 +260,8 @@ public void completed(final SSLSession sslSession) {
                     }
 
                 });
-        if (tlsSessionRef.compareAndSet(null, sslioSession)) {
-            currentSessionRef.set(ioSessionDecorator != null ? ioSessionDecorator.decorate(sslioSession) : sslioSession);
-        } else {
-            throw new IllegalStateException("TLS already activated");
-        }
+        tlsSessionRef.set(sslioSession);
+        currentSessionRef.set(ioSessionDecorator != null ? ioSessionDecorator.decorate(sslioSession) : sslioSession);
         try {
             if (sessionListener != null) {
                 sessionListener.startTls(sslioSession);