Gordon Brown has pledged to "get to the bottom" of "serious" allegations surrounding payments offered to four Labour peers in return for amendments to legislation, but urged commentators not to jump to conclusions.

The Prime Minister's comments came on the same day the Liberal Democrats called for a police investigation into the claims, which were made in the Sunday Times yesterday.

In the report, a journalist - posing as a businessman - approached four Labour Lords with cash sums - up to the value of £120,000 - offered in return for a change in laws.

The four Uppers Chamber members named in the report - Lords Truscott, Moonie, Snape and Taylor of Blackburn - have all denied any wrongdoing.

Lords Truscott and Moonie are both former cabinet ministers.

Speaking to assembled journalist this morning, the Prime Minister said: "It's important that we don't pre-judge these investigations but these are serious allegations and we are determined to get to the bottom of these allegations, and whatever action needs to be taken will be taken."

Lords Leader Baroness Royall has reportedly spoken to the four members at the centre of the allegation and is expected to meet again with the men later.

Peter Truscott, who was raised to the peerage in 2004, has admitted speaking to the reporter, but added: "To suggest I would offer to put down amendments for money is a lie."

Lewis Moonie, who was made a Baron in 2005, denied doing anything "outside the rules", but acknowledged discussing a fee of £30,000.

Long-serving Labour peer Thomas Taylor said he had been approached by two people offering him £5,000 - £10,000 per month as an adviser. However Lord Taylor said he had never agreed, signed no contract or took any money.

Peter Snape, who stood down as an MP in 2001 - and was subsequently elevated to the Lords - issued a statement reaffirming he was unable to "initiate or amend any legislation on behalf of an individual or a company".

Lib Dem Home Affairs spokesman Chris Huhne said he is to write to the Metropolitan Police over the claims.

"Our politics must be clean and seen to be clean in both the Commons and the Lords," he said.

Tory peer Lord Strathclyde branded the allegation "shocking".

The Lords Committee for Privileges - whose members include former MI5 boss Eliza Manningham-Buller and Lord Irvine, a former Lord Chancellor - has been commissioned to investigate the allegations made in yesterday's newspaper report.

(PR/BMcC)