BADSIG messages just started appearing on Ubuntu laptop

"BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key"

There's no obvious reason. However this fixed it.

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

 Thanks to NewInstance

--
This happened again in September !
Some bug perhaps ?