diff --git a/services/backup/src/http/handlers/backup.rs b/services/backup/src/http/handlers/backup.rs
--- a/services/backup/src/http/handlers/backup.rs
+++ b/services/backup/src/http/handlers/backup.rs
@@ -17,6 +17,7 @@
 use tokio_stream::{wrappers::ReceiverStream, StreamExt};
 use tracing::{info, instrument, trace, warn};
 
+use crate::identity::find_user_id;
 use crate::{
   database::{backup_item::BackupItem, DatabaseClient},
   error::BackupError,
@@ -282,8 +283,7 @@
   db_client: web::Data<DatabaseClient>,
 ) -> actix_web::Result<impl Responder> {
   let username = path.into_inner();
-  // Treat username as user_id in the initial version
-  let user_id = username;
+  let user_id = find_user_id(&username).await?;
 
   let Some(backup_item) = db_client
     .find_last_backup_item(&user_id)
@@ -308,8 +308,7 @@
   blob_client: Authenticated<BlobServiceClient>,
 ) -> actix_web::Result<HttpResponse> {
   let username = path.into_inner();
-  // Treat username as user_id in the initial version
-  let user_id = username;
+  let user_id = find_user_id(&username).await?;
 
   let Some(backup_item) = db_client
     .find_last_backup_item(&user_id)
diff --git a/services/commtest/tests/backup_integration_test.rs b/services/commtest/tests/backup_integration_test.rs
--- a/services/commtest/tests/backup_integration_test.rs
+++ b/services/commtest/tests/backup_integration_test.rs
@@ -75,10 +75,30 @@
     .await?;
   assert_eq!(user_data, backup_data.user_data);
 
+  // Test latest backup lookup for not-existing user
+  let latest_backup_descriptor = BackupDescriptor::Latest {
+    username: "non_existing_user".to_string(),
+  };
+
+  let non_existing_user_response = backup_client
+    .download_backup_data(&latest_backup_descriptor, RequestedData::BackupID)
+    .await;
+
+  match non_existing_user_response {
+    Ok(_) => panic!("Expected error, but got success response"),
+    Err(BackupClientError::ReqwestError(error)) => {
+      assert_eq!(
+        error.status(),
+        Some(StatusCode::BAD_REQUEST),
+        "Expected bad request status"
+      );
+    }
+    Err(_) => panic!("Unexpected error type"),
+  }
+
   // Test latest backup lookup
   let latest_backup_descriptor = BackupDescriptor::Latest {
-    // Initial version of the backup service uses `user_id` in place of a username
-    username: device_info.user_id.to_string(),
+    username: device_info.username,
   };
 
   let backup_id_response = backup_client
diff --git a/services/commtest/tests/backup_performance_test.rs b/services/commtest/tests/backup_performance_test.rs
--- a/services/commtest/tests/backup_performance_test.rs
+++ b/services/commtest/tests/backup_performance_test.rs
@@ -81,9 +81,15 @@
       let mut handlers = vec![];
       for user in &user_identities {
         let backup_client = backup_client.clone();
-        let descriptor = BackupDescriptor::Latest {
-          username: user.user_id.clone(),
+
+        let username = if user.user_id == device_info_1.user_id {
+          device_info_1.username.clone()
+        } else {
+          device_info_2.username.clone()
         };
+
+        let descriptor = BackupDescriptor::Latest { username };
+
         handlers.push(tokio::spawn(async move {
           let response = backup_client
             .download_backup_data(&descriptor, RequestedData::BackupID)
@@ -107,9 +113,14 @@
       let mut handlers = vec![];
       for user in &user_identities {
         let backup_client = backup_client.clone();
-        let descriptor = BackupDescriptor::Latest {
-          username: user.user_id.clone(),
+        let username = if user.user_id == device_info_1.user_id {
+          device_info_1.username.clone()
+        } else {
+          device_info_2.username.clone()
         };
+
+        let descriptor = BackupDescriptor::Latest { username };
+
         handlers.push(tokio::spawn(async move {
           backup_client
             .download_backup_data(&descriptor, RequestedData::UserKeys)