diff --git a/ee/app/services/projects/update_mirror_service.rb b/ee/app/services/projects/update_mirror_service.rb
index 291b2488cfc07a006a253030348dbfb97b567f63..59e6e65b761b7c57be8a002ae808733f4ab693da 100644
--- a/ee/app/services/projects/update_mirror_service.rb
+++ b/ee/app/services/projects/update_mirror_service.rb
@@ -4,6 +4,8 @@ module Projects
   class UpdateMirrorService < BaseService
     include Gitlab::Utils::StrongMemoize
 
+    TEMP_BRANCH_PREFIX = "update-mirror/"
+
     Error = Class.new(StandardError)
     UpdateError = Class.new(Error)
 
@@ -32,12 +34,16 @@ def execute
         project.fetch_mirror(forced: true, check_tags_changed: true)
       end
 
-      update_branches
+      update_branches(branch_prefix: TEMP_BRANCH_PREFIX)
 
       # Updating LFS objects is expensive since it requires scanning for blobs with pointers.
       # Let's skip this if the repository hasn't changed.
       update_lfs_objects if update_lfs_objects?(checksum_before)
 
+      update_branches do |name|
+        ::Branches::DeleteService.new(project, current_user).execute("#{TEMP_BRANCH_PREFIX}#{name}")
+      end
+
       # Running git fetch in the repository creates loose objects in the same
       # way running git push *to* the repository does, so ensure we run regular
       # garbage collection
@@ -70,14 +76,14 @@ def normalized_url(url)
       end
     end
 
-    def update_branches
+    def update_branches(branch_prefix: "", &block)
       local_branches = repository.branches.index_by(&:name)
 
       errors = []
       branches_to_create = {}
 
       repository.upstream_branches.each do |upstream_branch|
-        name = upstream_branch.name
+        name = "#{branch_prefix}#{upstream_branch.name}"
 
         next if skip_branch?(name)
 
@@ -96,6 +102,8 @@ def update_branches
             errors << e.message
           end
         end
+
+        yield(name) if block
       end
 
       result = ::Branches::CreateService.new(project, current_user).bulk_create(branches_to_create)