Three peers have been accused of agreeing to carry out Parliamentary work for payment.
Undercover Sunday Times reporters filmed the men appearing to offer to help a fake solar energy company.
Ulster Unionist Lord Laird and Labour's Lord Mackenzie of Framwellgate and Lord Cunningham deny wrongdoing.
The two Labour peers have been suspended from the party and Lord Laird has resigned the party whip pending an investigation.
In a separate investigation by the BBC's Panorama programme in conjunction with the Daily Telegraph, Lord Laird was also secretly filmed discussing a retainer to ask parliamentary questions.
The House of Lords code of conduct says peers must not act as paid advocates and "must not seek to profit from membership of the House by accepting or agreeing to accept payment for providing parliamentary advice or services".
Lord Laird is alleged to have told reporters posing as lobbyists for a fake company representing business interests in Fiji that he would be interested in accepting a retainer of £2,000 a month.
He told the Panorama undercover team that he would help start an All Party Parliamentary Group for Fiji. He indicated that he would use the promise of a trip to Fiji as a "bribe" in order to get people to join the group.
Lord Laird said: "I'll deny having said this, but it's a bribe. The sort of thing I can say to these guys. 'Look, you put that question down now. I thought you were interested in Fiji. Would you like to come down to it, y'know? I believe it's quite nice.' But that's why I can say that."
In a similar but separate investigation by the Sunday Times, Lord Laird told undercover reporters that, working together, peers could ask parliamentary questions for each other's clients, put down amendments in debates or write to ministers.
But later, in a statement to the BBC, Lord Laird said he had been "the subject of a scam" by journalists.
"This has led to allegations that I have broken the rules of the House of Lords," he said.
"I wish to make it clear that I did not agree to act as a paid advocate in any proceedings of the House nor did I accept payment or other incentive or reward in return for providing parliamentary advice or services."
He said that, although he had not broken any rules, he had referred himself "to the appropriate authorities".
The Ulster Unionist Party later announced that Lord Laird had resigned the party whip, pending the outcome of an investigation by Westminster authorities.
Mike Nesbitt, head of the UUP, said: "Any sensible, right-minded person watching that video clip would form an opinion and that is not the opinion I would want for the Ulster Unionist Party."
'Value for money'
The second peer, Lord Cunningham - a minister under Tony Blair - offered to write to Prime Minister David Cameron to push the solar energy company's supposed agenda.
He also offered to ask parliamentary questions and said he could "get other people to ask questions as well".
He was offered a monthly payment of £10,000, to which he replied: "Make that... £12,000 a month. I think we could do a deal on that."
He told undercover reporters he offered "value for money" because he could introduce them to senior members of all three of the main political parties.
"Knocking on doors, introductions and getting to see people, including if necessary the ministers - this is part of the package," he said.
In a statement sent to the Sunday Times, Lord Cunningham denied breaking any rules.
He said: "I deny any agreement to operate in breach of the House of Lords code of conduct and, in fact, recall that I made it clear that I would only operate within the rules."
He said his reference to "a fanciful £12,000 a month payment" was made to test his suspicion that he had been talking to journalists.
Lord (Brian) Mackenzie, a former chief superintendent for Durham police and president of the Police Superintendents Association, said he could arrange parties for paying clients - including on the terrace of the House of Lords - after being asked if this was possible.
"There is a rule that you shouldn't host a reception in parliament where you have a pecuniary interest. I thought that's bloody nonsense. Nonetheless... how would you get round that?
"I just say to a colleague who has nothing to do with it, 'would you host a function for me?'" he said.
But Lord Mackenzie told the BBC he was "quite happy" that he had not broken the rules.
He said the undercover reporters may well have offered him "so many thousands a month", but no figure was agreed on.
"In the event, when I went back to my office I checked the codes of conduct and I decided that it was getting a bit near the mark and I decided to decline the offer," he added.
He said so long as a peer's outside interests were properly declared and registered, it would be "nonsense" to prohibit them from asking questions and raising issues.
They brought specialist knowledge and expertise to the House of Lords, he said.
The Sunday Times said all three peers had told their undercover reporters at the time that they would declare any payment in the House of Lords register - but then they discussed ways to get around declaration of interest rules by asking colleagues to table questions and host events.
Labour said in a statement released after the peers' suspension: "The Labour Party expects the highest standards of its representatives and believes that they have a duty to be transparent and accountable at all times."
The party has called for urgent cross-party talks on the stalled introduction of a register.
Shadow defence secretary Jim Murphy said the latest claims made his toes curl "out of utter embarrassment".
People would be "utterly sickened" with actions showing there was one system for those who govern and another for those being governed, he added.
Shadow foreign secretary Douglas Alexander expressed anger that the good name of the endeavour of politics was "once again being smeared by what appears to be conduct that literally cannot be defended".
Cabinet Office minister Francis Maude suggested lobbying reform could get under way before the 2015 election.
Peter Facey, director of campaign group Unlock Democracy, said peers were paid a tax-free allowance of £300 a day, plus travel expenses to sit in the House of Lords - the equivalent of a £60,000 annual salary if they attended every time the House sat.
On Saturday, Panorama released footage - secretly filmed as part of the same investigation - of MP Patrick Mercer appearing to offer a Commons security pass to a fake Fijian firm that paid him £4,000 to ask parliamentary questions. He had resigned the Tory whip the previous day.
Mr Mercer said he had taken the money for consultancy work outside Parliament.
Panorama will be shown on BBC One at 21:00 BST on Thursday.