diff --git a/CHANGELOG.md b/CHANGELOG.md
index b59962506..90311134d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Fixed
 
 * Fixed spelling error in measurement unit "milliliter".
+* Add thread-safe synchronisation when merging and uncompressing a large EHR Extract bundle to avoid a race condition
+  occurring where multiple threads may attempt to merge and uncompress the same EHR Extract bundle at the same time.
+  This could result in a corrupted EHR Extract bundle.
 
 ## [3.3.1] - 2026-06-01
 
diff --git a/gp2gp-translator/src/main/java/uk/nhs/adaptors/pss/translator/task/COPCMessageHandler.java b/gp2gp-translator/src/main/java/uk/nhs/adaptors/pss/translator/task/COPCMessageHandler.java
index f445495ec..6667a331f 100644
--- a/gp2gp-translator/src/main/java/uk/nhs/adaptors/pss/translator/task/COPCMessageHandler.java
+++ b/gp2gp-translator/src/main/java/uk/nhs/adaptors/pss/translator/task/COPCMessageHandler.java
@@ -128,11 +128,12 @@ public void handleMessage(InboundMessage inboundMessage, String conversationId)
             checkAndMergeFileParts(inboundMessage, conversationId);
             nackAckPreparationService.sendAckMessage(payload, conversationId, migrationRequest.getLosingPracticeOdsCode());
 
-            // merge and uncompress large EHR message
-            if (inboundMessageMergingService.canMergeCompleteBundle(conversationId)) {
-                inboundMessageMergingService.mergeAndBundleMessage(conversationId);
+            synchronized (this) {
+                // merge and uncompress large EHR message
+                if (inboundMessageMergingService.canMergeCompleteBundle(conversationId)) {
+                    inboundMessageMergingService.mergeAndBundleMessage(conversationId);
+                }
             }
-
         } catch (WebClientRequestException | ConnectionException | MhsServerErrorException e) {
             throw e;
         } catch (ParseException | ValidationException | SAXException e) {