Natty Update warning error message

Hi,
I'm running Natty 11.04, (2.6.38-8-generic #42-Ubuntu SMP Mon Apr 11 03:31:50 UTC 2011 i686 i686 i386 GNU/Linux), while updating it I've got following Warning Error message:
Quote
W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://archive.canonical.com natty Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <email address hidden>
Unqoute

What should I do got it fixed?

Thanks in advance