diff --git a/README.md b/README.md
index ee026ea..1e161df 100644
--- a/README.md
+++ b/README.md
@@ -12,7 +12,7 @@ Lagom.js is built against specific versions of Lagom, the latest are:
 |-------------|-------|-----------------|----------|
 | 0.1.2-1.5.5 | 1.5.5 | 2.11 <br> 2.12  | 0.6.31   |
 | 0.3.2-1.6.2 | 1.6.2 | 2.12 <br> 2.13  | 0.6.33   |
-| 0.4.0-1.6.4 | 1.6.4 | 2.12 <br> 2.13  | 1.2.0    |
+| 0.4.1-1.6.4 | 1.6.4 | 2.12 <br> 2.13  | 1.2.0    |
 
 Lagom.js moved to Scala.js 1.x starting with version `0.4.0-1.6.2`. Scala.js 0.6 is no longer supported, the last version to support it was `0.3.2-1.6.2`. For all past releases, see [releases](#Releases).
 
@@ -27,7 +27,7 @@ Lagom.js provides JavaScript versions of several Lagom artifacts. The two most i
 The `lagomjs-scaladsl-api` artifact provides the JavaScript implementation of the Lagom service API:
 
 ```sbt
-"com.github.mliarakos.lagomjs" %%% "lagomjs-scaladsl-api" % "0.4.0-1.6.4"
+"com.github.mliarakos.lagomjs" %%% "lagomjs-scaladsl-api" % "0.4.1-1.6.4"
 ```
 
 To use it you'll need to configure your service API as a [Scala.js cross project](https://github.com/portable-scala/sbt-crossproject) for the JVM and JS platforms. Then, add the `lagomjs-scaladsl-api` dependency to the JS platform:
@@ -39,7 +39,7 @@ lazy val `service-api` = crossProject(JVMPlatform, JSPlatform)
     libraryDependencies += lagomScaladslApi
   )
   .jsSettings(
-    libraryDependencies += "com.github.mliarakos.lagomjs" %%% "lagomjs-scaladsl-api" % "0.4.0-1.6.4"
+    libraryDependencies += "com.github.mliarakos.lagomjs" %%% "lagomjs-scaladsl-api" % "0.4.1-1.6.4"
   )
 ```
 
@@ -50,7 +50,7 @@ This enables your Lagom service definition to be compiled into JavaScript. In ad
 The `lagomjs-scaladsl-client` artifact provides the JavaScript implementation of the Lagom service client:
 
 ```sbt
-"com.github.mliarakos.lagomjs" %%% "lagomjs-scaladsl-client" % "0.4.0-1.6.4"
+"com.github.mliarakos.lagomjs" %%% "lagomjs-scaladsl-client" % "0.4.1-1.6.4"
 ```
 
 You can use it in a Scala.js project along with your service API to generate a service client:
@@ -58,7 +58,7 @@ You can use it in a Scala.js project along with your service API to generate a s
 ```scala
 lazy val `client-js` = project
   .settings(
-    libraryDependencies += "com.github.mliarakos.lagomjs" %%% "lagomjs-scaladsl-client" % "0.4.0-1.6.4"
+    libraryDependencies += "com.github.mliarakos.lagomjs" %%% "lagomjs-scaladsl-client" % "0.4.1-1.6.4"
   )
   .enablePlugins(ScalaJSPlugin)
   .dependsOn(`service-api`.js)
@@ -80,6 +80,9 @@ However, the service client does not support a few the features available in Lag
 - subscribing to topics: topic definitions are available in the service client, but attempting to subscribe to the topic throws an exception
 - advanced service locators: service locators outside of the built-in service locators, such as `AkkaDiscoveryServiceLocator`, are not available
 
+## Known issues
+`ServiceCalls` based on akka-stream `Source` are implemented using Websockets, which don't propagate backpressure. This might change with [WebSocketStream API](https://web.dev/websocketstream/) once it's widely available.
+
 ## Releases
 
 Lagom.js tracks Lagom and generally doesn't continue development on previous Lagom releases. Since Lagom maintains a stable API within a minor release (e.g., 1.6.x) any version of Lagom.js built for that minor release should work. However, if you need Lagom.js for a specific previous Lagom release the previous Lagom.js releases are listed below. If you have an issue with a previous Lagom.js release please open an [issue](https://github.com/mliarakos/lagom-js/issues) and it will be considered on a case-by-case basis.
@@ -96,4 +99,5 @@ Lagom.js tracks Lagom and generally doesn't continue development on previous Lag
 | 0.4.0-1.6.2 | 1.6.2 | 2.12 <br> 2.13  | 1.0.1    |
 | 0.4.0-1.6.3 | 1.6.3 | 2.12 <br> 2.13  | 1.1.1    |
 | 0.4.0-1.6.4 | 1.6.4 | 2.12 <br> 2.13  | 1.2.0    |
+| 0.4.1-1.6.4 | 1.6.4 | 2.12 <br> 2.13  | 1.2.0    |
 
diff --git a/build.sbt b/build.sbt
index 3cfdda0..334cdca 100644
--- a/build.sbt
+++ b/build.sbt
@@ -50,7 +50,7 @@ lazy val publishSettings = Seq(
 
 lazy val commonSettings = scalaSettings ++ publishSettings ++ Seq(
   organization := "com.github.mliarakos.lagomjs",
-  version := s"0.4.0-$baseLagomVersion"
+  version := s"0.4.1-$baseLagomVersion"
 )
 
 lazy val commonJsSettings = Seq(
diff --git a/lagomjs-client-scaladsl/js/src/main/scala/com/lightbend/lagom/internal/scaladsl/client/ScaladslWebSocketClient.scala b/lagomjs-client-scaladsl/js/src/main/scala/com/lightbend/lagom/internal/scaladsl/client/ScaladslWebSocketClient.scala
index 2821e91..760001c 100644
--- a/lagomjs-client-scaladsl/js/src/main/scala/com/lightbend/lagom/internal/scaladsl/client/ScaladslWebSocketClient.scala
+++ b/lagomjs-client-scaladsl/js/src/main/scala/com/lightbend/lagom/internal/scaladsl/client/ScaladslWebSocketClient.scala
@@ -1,9 +1,11 @@
 package com.lightbend.lagom.internal.scaladsl.client
 
+import akka.stream.Materializer
 import com.lightbend.lagom.internal.client.WebSocketClient
+import com.typesafe.config.Config
 
 import scala.concurrent.ExecutionContext
 
-private[lagom] class ScaladslWebSocketClient()(implicit ec: ExecutionContext)
-    extends WebSocketClient()(ec)
+private[lagom] class ScaladslWebSocketClient(config : Config)(implicit ec: ExecutionContext, materializer: Materializer)
+    extends WebSocketClient(config)(ec, materializer)
     with ScaladslServiceApiBridge
diff --git a/lagomjs-client-scaladsl/js/src/main/scala/com/lightbend/lagom/scaladsl/client/ServiceClient.scala b/lagomjs-client-scaladsl/js/src/main/scala/com/lightbend/lagom/scaladsl/client/ServiceClient.scala
index 5f8c69c..c66f471 100644
--- a/lagomjs-client-scaladsl/js/src/main/scala/com/lightbend/lagom/scaladsl/client/ServiceClient.scala
+++ b/lagomjs-client-scaladsl/js/src/main/scala/com/lightbend/lagom/scaladsl/client/ServiceClient.scala
@@ -136,7 +136,8 @@ trait LagomServiceClientComponents extends TopicFactoryProvider { self: LagomCon
   lazy val serviceResolver: ServiceResolver                = new ScaladslServiceResolver(defaultExceptionSerializer)
   lazy val defaultExceptionSerializer: ExceptionSerializer = new DefaultExceptionSerializer(environment)
 
-  lazy val scaladslWebSocketClient: ScaladslWebSocketClient = new ScaladslWebSocketClient()(executionContext)
+  lazy val scaladslWebSocketClient: ScaladslWebSocketClient =
+    new ScaladslWebSocketClient(config)(executionContext, materializer)
   lazy val serviceClient: ServiceClient = new ScaladslServiceClient(
     scaladslWebSocketClient,
     serviceInfo,
diff --git a/lagomjs-client/js/src/main/scala/com/lightbend/lagom/internal/client/WebSocketClient.scala b/lagomjs-client/js/src/main/scala/com/lightbend/lagom/internal/client/WebSocketClient.scala
index 7693597..272a3c9 100644
--- a/lagomjs-client/js/src/main/scala/com/lightbend/lagom/internal/client/WebSocketClient.scala
+++ b/lagomjs-client/js/src/main/scala/com/lightbend/lagom/internal/client/WebSocketClient.scala
@@ -4,11 +4,16 @@ import java.net.URI
 import java.nio.ByteBuffer
 
 import akka.NotUsed
+import akka.stream.Materializer
+import akka.stream.OverflowStrategy
+import akka.stream.StreamDetachedException
 import akka.stream.scaladsl.Flow
+import akka.stream.scaladsl.Keep
 import akka.stream.scaladsl.Sink
 import akka.stream.scaladsl.Source
 import akka.util.ByteString
 import com.lightbend.lagom.internal.api.transport.LagomServiceApiBridge
+import com.typesafe.config.Config
 import org.reactivestreams.Publisher
 import org.reactivestreams.Subscriber
 import org.reactivestreams.Subscription
@@ -18,14 +23,21 @@ import org.scalajs.dom.WebSocket
 import org.scalajs.dom.raw.MessageEvent
 import play.api.http.Status
 
+import scala.collection.JavaConverters._
 import scala.concurrent.ExecutionContext
 import scala.concurrent.Future
 import scala.concurrent.Promise
 import scala.scalajs.js.typedarray.ArrayBuffer
 import scala.scalajs.js.typedarray.TypedArrayBuffer
 import scala.scalajs.js.typedarray.TypedArrayBufferOps._
+import scala.util.Failure
+import scala.util.Success
 
-private[lagom] abstract class WebSocketClient()(implicit ec: ExecutionContext) extends LagomServiceApiBridge {
+private[lagom] abstract class WebSocketClient(config: Config)(implicit ec: ExecutionContext, materializer: Materializer)
+    extends LagomServiceApiBridge {
+
+  // Defines the internal buffer size for the websocket when using ServiceCalls containing a Source as it's not possible to use backpressure in the JS websocket implementation
+  val maxBufferSize = Option(config.getInt("lagom.client.websocket.scalajs.maxBufferSize")).filter(_!=0).getOrElse(1024)
 
   /**
    * Connect to the given URI
@@ -119,50 +131,73 @@ private[lagom] abstract class WebSocketClient()(implicit ec: ExecutionContext) e
     private val NormalClosure   = 1000
     private var hasSubscription = false
 
+    // acts as a buffer between the backpressare unaware Websocket and backpressure aware reactive-streams Subscriber
+    val (messageSource, messageSink) = Source
+      .queue[ByteString](maxBufferSize, OverflowStrategy.fail)
+      .toMat(Sink.queue())(Keep.both)
+      .run()
+
+    // fill buffer even before a subscriber is connected, otherwise we might loose elements
+    socket.onmessage = { message =>
+      // The message data should be either an ArrayBuffer or a String
+      // It should not be a Blob because the socket binaryType was set
+      val data = message.data match {
+          case buffer: ArrayBuffer => ByteString.apply(TypedArrayBuffer.wrap(buffer))
+          case data                => ByteString.fromString(data.toString)
+        }
+
+      messageSource.offer(data).onComplete {
+        case Failure(exception) =>
+          throw exception
+        case _ =>
+      }
+    }
+
+    socket.onerror = event =>
+      messageSource.fail(new RuntimeException(s"WebSocket error: ${event.`type`}"))
+
+    // acting on the buffer instead of directly on the subscriber e.g. to make sure pending elements are delivered before completion
+    socket.onclose = event => {
+      if (event.code == NormalClosure) {
+        messageSource.complete()
+      } else {
+        // Complete the subscriber with an error because the socket closed with an error
+        // Parse the error reason as an exception
+        val bytes = ByteString.fromString(event.reason)
+        val exception =
+          exceptionSerializerDeserializeWebSocketException(exceptionSerializer, event.code, requestProtocol, bytes)
+        messageSource.fail(exception)
+      }
+    }
+
+    var completed = false
+
     override def subscribe(subscriber: Subscriber[_ >: ByteString]): Unit = {
       if (!hasSubscription) {
         hasSubscription = true
 
-        // Forward messages from the socket to the subscriber
-        val onMessage = (message: MessageEvent) => {
-          // The message data should be either an ArrayBuffer or a String
-          // It should not be a Blob because the socket binaryType was set
-          val data = message.data match {
-            case buffer: ArrayBuffer => ByteString.apply(TypedArrayBuffer.wrap(buffer))
-            case data                => ByteString.fromString(data.toString)
-          }
-          subscriber.onNext(data)
-        }
-        // Complete the subscriber with an error if the socket encounters an error
-        val onError = (event: Event) => {
-          subscriber.onError(new RuntimeException(s"WebSocket error: ${event.`type`}"))
-        }
-        // Complete the subscriber when the socket closes based on the close code
-        val onClose = (event: CloseEvent) => {
-          if (event.code == NormalClosure) {
-            // Successfully complete the subscriber because the socket closed normally
-            subscriber.onComplete()
-          } else {
-            // Complete the subscriber with an error because the socket closed with an error
-            // Parse the error reason as an exception
-            val bytes = ByteString.fromString(event.reason)
-            val exception =
-              exceptionSerializerDeserializeWebSocketException(exceptionSerializer, event.code, requestProtocol, bytes)
-            subscriber.onError(exception)
-          }
-        }
-
         // Configure the subscriber to start the stream
         subscriber.onSubscribe(new Subscription {
-          private var hasStarted = false
-          // Configure the socket after the initial subscriber request
           override def request(n: Long): Unit = {
-            if (!hasStarted) {
-              hasStarted = true
-              socket.onmessage = onMessage
-              socket.onerror = onError
-              socket.addEventListener[CloseEvent]("close", onClose)
+            // pull as many elements as requested, but only as long as element Source has not completed
+            def pull(): Unit = {
+              messageSink.pull().onComplete {
+                case Failure(_: StreamDetachedException) =>
+                // happens when subscriber requests more items than available or Subscription.request() is called before all requested elements have been onNext'ed
+
+                case Failure(exception) =>
+                  subscriber.onError(exception)
+
+                case Success(Some(data)) =>
+                  subscriber.onNext(data)
+                  pull()
+
+                case Success(None) =>
+                  subscriber.onComplete()
+                  completed = true
+              }
             }
+            if (!completed) pull()
           }
           // Close the socket if the subscriber cancels
           override def cancel(): Unit = socket.close()