GPG error: http://archive.ubuntu.com lucid Release: The following signatures were invalid

If running 'apt-get update' you get the error:

Reading package lists... Done
W: GPG error: http://archive.ubuntu.com lucid Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key

then

cd /var/lib/apt
mv lists lists.old
mkdir -p lists/partial
apt-get clean
apt-get update