Two former UK foreign secretaries, Jack Straw and Sir Malcolm Rifkind, have said they have broken no rules after being secretly filmed apparently offering their services to a private company for cash in a 'sting' operation conducted by reporters for the Daily Telegraph and Channel 4's Dispatches, who posed as staff of a fake Chinese firm.
Straw said he had fallen into a "very skilful trap", while Sir Malcolm said his comments had been "silly".
The MPs have referred themselves to Parliament's standards watchdog.
Labour leader Ed Miliband has written to Prime Minister Edward Cameron calling for a ban on MPs having second jobs.
Straw was recorded describing how he operated "under the radar" and had used his influence to change EU rules on behalf of a firm which paid him £60,000 a year.
On the subject of payment, Straw is heard saying, "So normally, if I'm doing a speech or something, it's £5,000 a day, that's what I charge."
Rifkind is reported to have claimed he could arrange "useful access" to every British ambassador in the world.
The Conservative MP for Kensington and chairman of Parliament's Intelligence and Security Committee was recorded saying, "I am self-employed - so nobody pays me a salary. I have to earn my income."
He said his usual fee for half a day's work was "somewhere in the region of £5,000 to £8,000".
Straw has been suspended from the Parliamentary Labour Party at his own request. But Sir Malcolm is not facing any suspension.
He told a BBC correspondent that the allegations had no bearing on his very significant role as chairman of the parliamentary committee that oversees the work of MI5 and MI6.
Downing Street has not offered any view on that. A source said it is for the Commons and other members of that committee to decide.
Both men defended themselves on appearances on BBC Radio 4's Today programme on Monday morning.
Rifkind said he had "nothing to be embarrassed about". He said the allegations were "unfounded" and he vowed to fight them "with all my strength".
He said he had never accepted an offer from the fake firm, saying it was a "preliminary" discussion "about what they had mind".
Sir Malcolm is paid £67,000 a year and he said telling the company he was not paid a salary was a "silly thing to say".
"Of course I receive a salary as a Member of Parliament but I was referring to my business interests, from none of which I receive a salary. I receive payment for services I provide," he said.