Labour's leader in the House of Lords today promised a full investigation of allegations that four of the party's peers agreed to seek amendments to legislation in return for cash fees of up to £10,000 a month.
Baroness Royall of Blaisdon said it would be "very grim" if the allegations proved to be correct, but stressed that she wanted to hear the accounts of the four peers, all of whom deny wrongdoing, before coming to any judgment.
If they were found to have broken the rules of the Upper House, they could be "named and shamed" in the Lords, but cannot be expelled from Parliament, she said.
Lady Royall said that the peers appear to have been "entrapped" by undercover reporters posing as lobbyists working for business clients.
The Sunday Times named four Labour peers who it claimed offered to help its reporters for a fee, two of whom the paper said were secretly recorded.
The House of Lords Code of Conduct states that peers "must never accept any financial inducement as an incentive or reward for exercising parliamentary influence".
As the allegations emerged last night, Baroness Royall said she was "deeply concerned" and promised to look into them "with the utmost vigour".
Liberal Democrat MP Norman Baker said he would raise the claims with the House authorities, saying: "Legislators in the Commons and the Lords are there to pass legislation on behalf of the country, not to change the law in return for financial favours."
But two of the peers named by the paper told the Press Association that they did not believe they had done anything wrong.
The Sunday Times said that its reporters approached Lord Taylor of Blackburn claiming to be acting on behalf of a Hong Kong businessman who was concerned at the impact of the Business Rate Supplements Bill on his company.
The paper said Lord Taylor agreed to work "behind the scenes" to try to ensure that the bill was amended and negotiated a £120,000 annual fee.
Lord Taylor told PA he was approached by two people claiming to work for a lobbying firm and looking for help with a bill they wanted amending. He said that they had suggested taking him on as an adviser at a fee of between £5,000 to £10,000 a month.
"It was their suggestion, not my suggestion," he said. "I never said I would accept it."
No contract was signed and no money changed hands, he said.
The paper also said that former defence minister Lord Moonie offered to assist its reporters in return for an annual fee of £30,000.
Lord Moonie last night said he had an "informal conversation" after which a ballpark figure was suggested of what it might cost if they were to take his advice.
But he insisted: "No contract was offered and I have not had a chance to speak to the registrar about it, as I would have done had I intended to take on work of this kind."
Lady Royall told BBC1's Andrew Marr Show: "For the moment these are allegations and I want to ensure that they are properly investigated. Once we have seen clearly what the issues are, then we will see whether or not action is necessary.
"These people were entrapped. If it is true, it is a very grim picture, but we do have to look into the details very carefully... Clearly it looks very serious, but I am concerned that everybody's side of the story should be heard."
Lady Royall said that, if the allegations were proved correct, it might be necessary to change the rules on lobbying in the House of Lords to bring them more closely in line with the tighter controls in place in the Commons.
"These things we are constantly looking at because we want to ensure that we have the highest levels of probity," she said. "We want to ensure that the high standards are there and are adhered to."
The Sunday Times also said it had taped Labour peer Lord Truscott of St James offering to work "behind the scenes" to facilitate an amendment to legislation, and suggesting that the fee for such assistance would be between £1,000 and £5,000 a day.
And the paper said that former Labour whip Lord Snape indicated he would be willing to help for a fee of up to £24,000 a year.
A woman who answered the phone at Lord Snape's Birmingham home this morning said he was not available to comment on the allegations, while there was no reply at Lord Truscott's address in central London.
Liberal Democrat leader Nick Clegg said that the allegations highlighted the need for the Lords to be reformed "completely, from top to toe".
Mr Clegg told BBC1's Andrew Marr Show: "I think people will be very, very angry reading about these allegations.
"This is the week when people are filling in their tax forms, every family in the country is worrying about how they spend their own money, and what do they hear from Parliament? Firstly, a Government seeking to cover up how MPs use their expenses, paid for by the taxpayer, and secondly allegations of total corruption in the House of Lords.
"Now we need to investigate whether these allegations are true, but if anybody needed any reminder of how threadbare and weak the credibility of Westminster and the way we do politics has become, you just need to look at today's headlines.
"I think it is a scandal that both Labour and the Conservative Party for so many years have simply refused to make the profound changes we need to make to bring British politics into the 21st century."
Shadow business secretary Ken Clarke said if the allegations were true, they amounted to "corruption".
He told BBC1's Andrew Marr Show: "I think the Commissioner for Standards has got to carry out an independent inquiry very rapidly.
"The men involved have got to have a chance to defend themselves and the allegations may be untrue.
"If the allegations are true, I'm afraid this one is very serious - to take money to try to alter legislation for the benefit of the people paying you a fee ... I think some people would call that corruption."