Pressure is mounting for a sleaze probe after several senior Labour MPs were filmed apparently offering their expertise for cash.
There has been cross-party condemnation 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.
"These are shocking allegations. First of all the House of commons needs to conduct a thorough investigation into these (former) Labour ministers," he said. "But also the Prime Minister would want to get to the bottom of the accusations being made about his Government."
All of the MPs filmed, including former Cabinet ministers Stephen Byers, Patricia Hewitt and Geoff Hoon, denied any wrongdoing and insisted they had breached no rules.
But Cabinet ministers said the behaviour of former 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. 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.