By Ian Dunt Follow @IanDunt

David Cameron and Alastair Campbell were at loggerheads today over the prime minister's assertion that the former director of communications had falsified documents.

Mr Cameron made the allegation during a heated exchange with Ed Balls in the wake of yesterday's phone-hacking statement in the Commons.

"Things can get very heated in the Commons, and things get said which shouldn’t be, but our system does depend on the public trusting ministers to tell the truth at the despatch box, and what Mr Cameron said is not true, and he knows it is not true," Mr Campbell wrote on his blog.

"I have a lot of respect for anyone who puts their head above the parapet and goes high up in politics and despite our very different beliefs and values, that includes David Cameron, because I know what a tough job he has. But he cannot make statements like the one he made yesterday, under parliamentary privilege, and expect me just to sit there and take it."

The accusation presumably related to accusations that Mr Campbell 'sexed-up' a dossier on Iraq in the run-up to the war.

The Hutton inquiry largely exonerated the former director of communications at Downing Street, but most commentators branded it a whitewash.

Mr Campbell has written to the Cabinet secretary asking for the government to produce evidence to substantiate the claim.

MPs cannot be sued for libel while making statements in the Commons due to parliamentary privilege.