Labour was left red-faced today after being forced to deny that a senior MP would be rewarded for supporting Gordon Brown's controversial plan to hold terror suspects for 42 days.

Keith Vaz, chairman of the Home Affairs committee, and Geoff Hoon, the Labour Chief Whip, rejected any suggestion of impropriety after a letter from Mr Hoon stating that he hoped Mr Vaz would be “appropriately rewarded” was disclosed to the Commons by David Cameron.

Both insisted that the sentiment in the letter was light-hearted. There had been much gossip, rumour and joking around the time of the key vote three weeks ago that Mr Vaz would be knighted or ennobled for backing 42 days, after receiving concessions from ministers. His committee had issued a report that was sceptical about the plan.

Most MPs accepted that Mr Hoon was joking in his letter and believed it unthinkable that the Chief Whip would have committed such an idea to paper.

But there was mystery over how it had come to be leaked, and its appearance was ammunition for Mr Cameron as he continues to claim that Gordon Brown’s victory over 42 days was achieved only by grubby deals between the Government and the nine MPs from Northern Ireland's Democratic Unionist party.

During Prime Minister's Questions, the Conservative leader referred to a handwritten letter from Mr Hoon to Mr Vaz. In the letter, sent on June 12, 2008 — the day after the key vote — Mr Hoon writes: "Dear Keith . . . Just a quick note to thank you for all your help during the period leading up to last Wednesday's vote. I wanted you to know how much I appreciated all your help.

"I trust that it will be appropriately rewarded! . . . With thanks and best wishes, Geoff."

Mr Cameron asked Mr Brown to confirm what he had previously said on the 42 days vote — that no deals were done, no jobs were offered and no rewards were promised. When Mr Brown replied: "Yes", Mr Cameron brought up the letter and asked what Mr Hoon had meant by "appropriately rewarded".

The Prime Minister explained: "That we thanked the chairman of the Home Affairs committee for doing exactly the right thing . . . voting with the Government."

Mr Vaz, who denied rumours that he had been offered a knighthood during the Commons debate leading up to the vote on terror detention limits, told the BBC that the letter was "totally light-hearted" and said that it was normal for whips to write to backbenchers after important votes.

He said the row showed that Mr Cameron was "clutching at straws". He dismissed as "ridiculous" suggestions that he was offered a peerage or a knighthood to vote with the Government.

An aide to Mr Hoon said that it was a private letter between two old friends. The expression of gratitude was sincere, but the remark about the reward was light hearted, as the exclamation mark showed. “We vigorously deny that that was a serious remark, or any suggestion that Mr Vaz would be rewarded in any way. It was ironic, tongue-in-cheek, a joke. Anyone who reads it can see that.”

Hazel Blears, the Communities Secretary, told the Daily Politics programme on BBC 2: "If you look at Keith Vaz's position on the 42 days he was always going to support it — this wasn't a case of, you know, 'If you vote with us, we will offer you this'."

She said that the Government had moved closer to the suggestions made by the Home Affairs Select Committee, in "significant changes" made to the Counter Terrorism Bill.

David Davis, the Shadow Home Secretary, who resigned his seat to fight a by-election after the 42-day vote, called on the Prime Minister to explain Mr Hoon’s comments.

“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.

In addition to alleged pledges to Mr Vaz, there have been claims of a wide variety of promises made to MPs if they backed the government in the vote. Other offers were said to include British support for a relaxation of the embargo against Cuba and better compensation for sick former coal miners.

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