Guardian and Google have determined that you may have what is termed a “conflicting account” in the Google universe.
Conflicting accounts exist if you created a personal Google Account using a Guardian email address. Because this email address will exist in your new Guardian Google account, it will need to be removed from your personal Google account and a new address inserted. Conflicting accounts most commonly occur if you were logging into Picasa Web Albums, Blogger, or any of these other Google Applications, before Guardian implemented Gmail.
Please confirm if you have a conflicting account. If so, please follow this link to How to Fix a Conflicting Account.
Going Google Project Team