Downing Street says it has no evidence that the PM received a letter from the Conservative Party warning that an MP's conversations may have been bugged.
Newspapers have alleged that Labour MP Sadiq Khan's discussions with his constituent Babar Ahmad in prison were monitored by anti-terrorist officers.
Shadow home secretary David Davis says he wrote to the PM in December, but No 10 says it has no record of the letter.
A government inquiry is now under way into the prison bugging claims.
Mr Ahmad faces no charges in the UK but is awaiting a decision from the European courts to discover if he will be extradited to the US, on suspicion of running websites raising funds for the Taleban.
Police are entitled to monitor prisoners' communications, but such operations must first be approved by the surveillance commissioner who considers them on a case-by-case basis.
The Sunday Times claimed Mr Khan's visits to Mr Ahmad in jail in Milton Keynes in 2005 and 2006, were bugged by Scotland Yard's anti-terrorist branch. Scotland Yard has refused to comment on the allegation.
The bugging of MPs has been barred since eavesdropping scandals during Harold Wilson's government.
Justice Secretary Jack Straw said he had no knowledge of the facts surrounding the Khan case but added: "It is completely unacceptable for an interview to be conducted by a MP, on a constituent matter or in any other issue, to be recorded."
The Commons home affairs committee is also investigating the claims as part of its wider inquiry into whether the UK is becoming a "surveillance society".
The Conservatives have published a letter which they say Mr Davis sent to Gordon Brown on 11 December. It asks him whether the Wilson Doctrine - established in 1966 which states that MPs will not be bugged - is still in place.
The letter reads: "I have reason to believe that the policy has been breached in relations to communications between a constituent, arrested and detained as a terrorist suspect since 2003, and his member of Parliament.
"I would be grateful if you could review this serious matter and inform me of the outcome of your review."
HAVE YOUR SAY
Given the way some MPs act in this day and age I'd say it was about time the law was changed to allow bugging of their conversations.
A spokeswoman for Downing Street said a number of letters from Mr Davis had been received since December on various issues.
She added: "But having conducted a detailed check, we have no record of receiving a letter about the bugging of MPs.
"Consequently the prime minister knows nothing about this letter."
Mr Davis told the BBC he would be calling for a Commons statement on the issue on Monday and said it needed to be established who had made the decision.
Mr Davis said he had heard about the bugging allegations from "a source" but added: "It was somebody who was concerned that the Wilson doctrine was being broken and I thought the only proper thing to do was to notify the prime minister in confidence, which is what I did.
The US is trying to extradite Babar Ahmad
"I have to say I'm rather surprised that nothing has happened in the interim and indeed that the Justice Secretary, Jack Straw, is claiming that this is some sort of surprise."
Earlier Mr Khan, a government whip and Labour MP for Tooting, told the BBC he welcomed Mr Straw's announcement that there would be an inquiry, and said he did not know if the allegations that his visits were bugged were true.
But he added: "If it was the case...that constituents were having their conversations with members of Parliament bugged, then it clearly means the whole relationship and the basis on which that relationship has been based for decades and, dare I say centuries, is undermined."
In a statement, the Metropolitan Police said: "We do not discuss matters of national security but recognise they are subject to proper scrutiny."
It said it would "encourage the use" of the independent Investigatory Powers Tribunal to investigate any complaints - and would "always would fully co-operate with the tribunal's work".