diff --git a/server/implementation/src/main/java/io/smallrye/graphql/websocket/AbstractGraphQLWebsocketHandler.java b/server/implementation/src/main/java/io/smallrye/graphql/websocket/AbstractGraphQLWebsocketHandler.java
index 019c2e320..534b85fa3 100644
--- a/server/implementation/src/main/java/io/smallrye/graphql/websocket/AbstractGraphQLWebsocketHandler.java
+++ b/server/implementation/src/main/java/io/smallrye/graphql/websocket/AbstractGraphQLWebsocketHandler.java
@@ -68,6 +68,7 @@ public void onThrowable(Throwable t) {
     @Override
     public void onClose() {
         LOG.debug("GraphQL-over-websocket session " + session + " closed");
+        activeOperations.forEach((id, operation) -> cancelOperation(id));
         if (!session.isClosed()) {
             session.close((short) 1000, "");
         }
@@ -200,11 +201,8 @@ private void sendKeepAlive() {
 
     protected void sendCancelMessage(JsonObject message) {
         String opId = message.getString("id");
-        Subscriber<ExecutionResult> subscriber = activeOperations.remove(opId);
-        if (subscriber != null) {
-            if (subscriber instanceof SubscriptionSubscriber) {
-                ((SubscriptionSubscriber) subscriber).cancel();
-            }
+        boolean cancelled = cancelOperation(opId);
+        if (cancelled) {
             if (LOG.isDebugEnabled()) {
                 LOG.debug("Completed operation id " + opId + " per client's request");
             }
@@ -217,6 +215,20 @@ protected void sendCancelMessage(JsonObject message) {
         }
     }
 
+    // cancel the operation with this id, returns true if it actually cancels an operation,
+    // false if no such operation is active
+    private boolean cancelOperation(String opId) {
+        Subscriber<ExecutionResult> subscriber = activeOperations.remove(opId);
+        if (subscriber != null) {
+            if (subscriber instanceof SubscriptionSubscriber) {
+                ((SubscriptionSubscriber) subscriber).cancel();
+            }
+            return true;
+        } else {
+            return false;
+        }
+    }
+
     private boolean validSubscription(String operationId) {
         if (!connectionInitialized.get()) {
             closeDueToConnectionNotInitialized();
@@ -293,7 +305,10 @@ public void onComplete() {
         }
 
         public void cancel() {
-            subscription.get().cancel();
+            Subscription sub = subscription.get();
+            if (sub != null) {
+                sub.cancel();
+            }
         }
     }
 
diff --git a/server/integration-tests/src/test/java/io/smallrye/graphql/tests/subscription/SubscriptionHasToBeCancelledOnWebSocketCloseTestCase.java b/server/integration-tests/src/test/java/io/smallrye/graphql/tests/subscription/SubscriptionHasToBeCancelledOnWebSocketCloseTestCase.java
new file mode 100644
index 000000000..f2e94887c
--- /dev/null
+++ b/server/integration-tests/src/test/java/io/smallrye/graphql/tests/subscription/SubscriptionHasToBeCancelledOnWebSocketCloseTestCase.java
@@ -0,0 +1,103 @@
+package io.smallrye.graphql.tests.subscription;
+
+import java.lang.reflect.Field;
+import java.net.URL;
+import java.time.Duration;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import org.eclipse.microprofile.graphql.GraphQLApi;
+import org.eclipse.microprofile.graphql.Query;
+import org.jboss.arquillian.container.test.api.Deployment;
+import org.jboss.arquillian.junit.Arquillian;
+import org.jboss.arquillian.test.api.ArquillianResource;
+import org.jboss.shrinkwrap.api.ShrinkWrap;
+import org.jboss.shrinkwrap.api.asset.EmptyAsset;
+import org.jboss.shrinkwrap.api.spec.WebArchive;
+import org.junit.Assert;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import io.smallrye.graphql.api.Subscription;
+import io.smallrye.graphql.client.Response;
+import io.smallrye.graphql.client.dynamic.api.DynamicGraphQLClient;
+import io.smallrye.graphql.client.vertx.dynamic.VertxDynamicGraphQLClient;
+import io.smallrye.graphql.client.vertx.dynamic.VertxDynamicGraphQLClientBuilder;
+import io.smallrye.mutiny.Multi;
+import io.vertx.core.http.HttpClient;
+
+/**
+ * Verify that subscription gets cancelled properly on the server side when a client websocket connection
+ * is closed, even though the client didn't send a COMPLETE message for the subscription.
+ *
+ * This means that the internally created subscriber of the server-side Multi has
+ * to receive an `onCancellation` callback.
+ */
+@RunWith(Arquillian.class)
+public class SubscriptionHasToBeCancelledOnWebSocketCloseTestCase {
+
+    @Deployment
+    public static WebArchive deployment() {
+        return ShrinkWrap.create(WebArchive.class, "dependent-scope-test.war")
+                .addAsWebInfResource(EmptyAsset.INSTANCE, "beans.xml")
+                .addClasses(Foo.class);
+    }
+
+    @ArquillianResource
+    URL testingURL;
+
+    @GraphQLApi
+    public static class Foo {
+
+        static AtomicBoolean CANCELLED = new AtomicBoolean(false);
+
+        @Subscription
+        public Multi<Long> counting() {
+            return Multi.createFrom()
+                    .ticks()
+                    .every(Duration.ofSeconds(1L))
+                    .onCancellation().invoke(() -> {
+                        // this has to be called on the internal subscriber when a client's websocket connection ends
+                        CANCELLED.set(true);
+                    });
+        }
+
+        @Query
+        public String dummy() {
+            return null;
+        }
+
+    }
+
+    @Test
+    public void testSubscriptionCancel() throws InterruptedException, NoSuchFieldException, IllegalAccessException {
+        DynamicGraphQLClient client = new VertxDynamicGraphQLClientBuilder()
+                .url(testingURL.toString() + "graphql")
+                .build();
+        Multi<Response> multi = client.subscription("subscription {counting}");
+        multi.subscribe().with(response -> {
+            System.out.println(response);
+        }, error -> {
+            error.printStackTrace();
+        });
+
+        // wait until some items are produced
+        TimeUnit.SECONDS.sleep(3);
+
+        System.out.println("Forcibly closing the websocket....");
+        closeUnderlyingHttpClient(client);
+
+        TimeUnit.SECONDS.sleep(1);
+        // assert that the internal subscriber of the Multi on the server side was closed
+        Assert.assertTrue("The server-side Multi's internal subscriber has to be cancelled " +
+                "after the client websocket connection is closed",
+                Foo.CANCELLED.get());
+    }
+
+    private void closeUnderlyingHttpClient(DynamicGraphQLClient client) throws NoSuchFieldException, IllegalAccessException {
+        Field httpClientField = VertxDynamicGraphQLClient.class.getDeclaredField("httpClient");
+        httpClientField.setAccessible(true);
+        HttpClient httpClient = (HttpClient) httpClientField.get(client);
+        httpClient.close();
+    }
+}