One of the former ministers embroiled in the lobbying row said he had asked Parliament's sleaze watchdog to investigate his conduct.
Ex-transport secretary Stephen Byers said he had referred himself to John Lyon and believed he would be cleared of any wrongdoing.
The move came as pressure mounted over allegations that senior Labour figures had offered to use their influence and contacts in return for cash.
Mr Byers said: "I am confident that (Mr Lyon) will confirm that I have complied with the MPs' code of conduct and have fully disclosed my outside interests."
There was cross-party condemnation on Sunday of former ministers who were caught in an undercover "sting" operation for a television documentary.
The revelations forced Labour to rush forward a promise to enforce a compulsory register of lobbying which it said had been planned for the election manifesto.
But Tory leader David Cameron said the case raised wider questions about whether the MPs and the serving Cabinet ministers they were alleged to have influenced had breached rules.
All of the MPs filmed, including former Cabinet ministers Mr Byers, Patricia Hewitt and Geoff Hoon, denied any wrongdoing and insisted they had breached no rules. But serving ministers said the behaviour of their colleagues had been "appalling" and "ridiculous", and Liberal Democrat leader Nick Clegg called it "very, very sleazy".
Mr Byers was among retiring MPs interviewed by an undercover reporter posing as the representative of a fictitious US lobbying firm. He told the undercover reporter he had secured secret deals with ministers, could get confidential information from Number 10 and was able to help firms involved in price-fixing get around the law.
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The Sunday Times, which carried out the interviews with Channel 4's Dispatches programme, said Mr Byers, who held several key Cabinet portfolios such as trade and transport, wanted £5,000 a day. The North Tyneside MP retracted his claims the following day - insisting he had "never lobbied ministers on behalf of commercial interests" and had exaggerated his influence.
