rubygems.diff

diff --git a/lib/rubygems.rb b/lib/rubygems.rb
index ca8032645..4e0d3971c 100644
--- a/lib/rubygems.rb
+++ b/lib/rubygems.rb
@@ -1188,6 +1188,8 @@ An Array (#{env.inspect}) was passed in from #{caller[3]}
   # other.
   class << self
 
+    attr_accessor :disable_system_update
+
     ##
     # Hash of loaded Gem::Specification keyed by name
 
diff --git a/lib/rubygems/commands/update_command.rb b/lib/rubygems/commands/update_command.rb
index 494b3eef6..cfb9e4e22 100644
--- a/lib/rubygems/commands/update_command.rb
+++ b/lib/rubygems/commands/update_command.rb
@@ -262,6 +262,11 @@ command to remove old versions.
   # Update RubyGems software to the latest version.
 
   def update_rubygems
+    if Gem.disable_system_update
+      alert_error Gem.disable_system_update
+      return
+    end
+
     check_update_arguments
 
     version, requirement = rubygems_target_version