Gordon Brown faced questions today over why his Chief Whip apparently assured a Labour MP he would be "appropriately rewarded" for his support in a crunch Commons vote on 42-day pre-charge detention.
The Prime Minister was challenged in the Commons after it was reported that Geoff Hoon had written a 'Thank you' message to Keith Vaz after winning the vote narrowly last month.
Mr Vaz, chairman of the influential Home Affairs Select Committee, backed the measure despite earlier opposition.
According to the Daily Telegraph, Mr Hoon wrote: "Just a quick note to thank you for all your help during the period leading up to last Wednesday's vote... I trust that it will be appropriately rewarded!"
Ministers have repeatedly denied doing deals with Labour rebels or the nine Democratic Unionist MPs whose votes proved pivotal.
Tory leader David Cameron challenged Mr Brown in the Commons during their weekly showdown: "Don't take people for fools. Tell us the truth. What did he mean?"
Mr Brown told the Commons today: "He was thanking the chairman of the Home Affairs Select Committee for doing exactly the right thing."
Mr Brown avoided a humiliating defeat over the extension of pre-charge detention for terror suspects by just nine votes last month.
It followed weeks of negotiations and arm-twisting, including concessions over the scrutiny and limits of the powers, up until the hours before the vote.
It was eventually won by just nine votes after 36 Labour MPs rebelled.
Before Mr Cameron raised Mr Hoon's letter over the Despatch Box, the Prime Minister today repeated his insistence that no back-room deals had been made to win over opponents.
There have been claims of a wide variety of promises made to MPs, including the offer of a peerage or knighthood for Mr Vaz.
Other offers allegedly included British support for a relaxation of the embargo against Cuba and compensation for coal miners.
Mr Brown's victory in the vote was a much-needed boost and led to the resignation of shadow home secretary David Davis.
Mr Davis is now fighting a by-election for his own seat - Haltemprice and Howden - on a civil liberties platform.
Mr Davis also called on the Prime Minister to explain Mr Hoon's comments in the letter.
"This is yet further evidence demonstrating the grubby deal-making it took for Gordon Brown to get 42 days through the House of Commons, despite widespread opposition across the political parties," he said.
"It is now incumbent upon Mr Brown to immediately explain precisely what his Chief Whip meant when he said the chairman of the Home Affairs Select Committee, Keith Vaz, would be 'appropriately rewarded' for reversing his opposition to 42 days, and voting with the Government."