Alastair Campbell has accepted responsibility for the Labour advertisement which portrayed Michael Howard and Oliver Letwin as flying pigs.

In an exchange of emails with a Newsnight producer, Campbell initially told the programme to "fuck off and cover something important you twats."

Later the programme received another email in which he claimed that the first had been meant for someone else, and said that he had "dreamt up" the ad.

BBC sources are said to be confident that the first email was meant for them.