The Leader of the House of Lords Lady Royall is to make a statement on claims four Labour peers were ready to take cash to change legislation.
Two of the Labour peers at the centre of claims they were ready to take cash to change legislation have responded to the allegations in the House of Lords.
Lady Royall, who is meeting the four peers, will make a statement at about 1510 GMT, after pressure from the Conservatives and Liberal Democrats.
Lord Snape stood up to refute the allegations made by the Sunday Times.
Gordon Brown said he was "determined to get to the bottom" of the allegations.
And Lord Taylor of Blackburn said he apologised if he had done anything to bring the House into disrepute, but said he felt he had followed the rules.
The Lib Dems say they are planning to make a complaint to the police over the Sunday Times claims.
Lords leader Lady Royall called the claims "deeply shocking" and said a "rigorous" inquiry had already begun.
The newspaper said peers had offered to help make amendments for up to £120,000.
She pointed out that they were only allegations at this stage but said the claims had been "damaging not just to this House but to Parliament and politics".
'Don't pre-judge'
The prime minister said the allegations were "serious" and "whatever action needs to be taken will be taken".
All four peers, who spoke to undercover reporters, have denied any wrongdoing.
Those named in the Sunday Times story are former energy minister Lord Truscott, former defence minister Lord Moonie, Lord Taylor of Blackburn and former Labour whip Lord Snape.
We need to get to the bottom of this and make sure the public realise that this is not just a bunch of politicians being judge and jury in their own courts Chris Huhne, Liberal Democrats Q&A: Peers cash claimsProfile: Peers in cash row
Mr Brown, taking part in a question-and-answer session at the Foreign Press Association in London, 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."
Lord Strathclyde, the Tory Leader in the Lords, told the BBC the allegations were "shocking" and said there needed to be an urgent investigation.
And Conservative Party chairman Eric Pickles told BBC Two's Daily Politics: "These are pretty serious allegations - if they are proved true, I expect these men not to be Labour peers."
'Utmost vigour'
Meanwhile the Lib Dems have tabled a Private Notice Question in the House of Lords: "To ask the Leader of the House what action she proposes to take in consequence of the allegations of misconduct in yesterday's Sunday Times."
The leader of the House of Lords, Lady Royall, has spoken to the peers and is meeting them later.
Lord Truscott did admit to having had "discussions" with a reporter, but said that "to suggest I would offer to put down amendments for money is a lie".
Gordon Brown: 'Peer claims very serious'
Lord Moonie acknowledged discussing a fee of £30,000 but said he had not done anything "outside the rules".
Lord Taylor of Blackburn said two people had approached him and suggested paying him £5,000 to £10,000 a month as an adviser, but that he never said he would accept, no contract was signed and no money changed hands.
Lord Snape issued a statement saying he had made it clear he was unable to "initiate or amend any legislation on behalf of an individual or a company".
'Judge and jury'
Lady Royall, who said she would be "pursuing the matter with utmost vigour", has asked the Lords Committee for Privileges to investigate.
Its members include former lord chancellor Lord Irvine of Lairg and Baroness Manningham-Buller, former director of MI5.
There have been calls for a wider look at the rules, which currently allow peers to be paid as consultants, as long as they do not take money for exercising parliamentary influence, our correspondent added.
Two separate parliamentary inquiries are also expected to look at the issues surrounding the four peers.
If the peers are found guilty of having broken the rules, they could be required to apologise on the floor of the Lords, but cannot be expelled from Parliament or stripped of their titles.
Lib Dem home affairs spokesman Chris Huhne is to write to the Metropolitan Police over the claims, telling the BBC: "We need to get to the bottom of this and make sure the public realise that this is not just a bunch of politicians being judge and jury in their own courts."
He said procedures in the Lords should be tightened to match those in the Commons, including the power to suspend or expel misbehaving members.
Sir Christopher Kelly, chairman of the Committee on Standards in Public Life, said he would await the outcome of the House of Lords investigation to see whether it was "an area" the watchdog needed to look at further.
The Sunday Times said its reporters had posed as lobbyists acting for a foreign client, who was setting up a chain of shops in the UK and wanted an exemption from the Business Rates Supplements Bill.
