The Tory leader joined cross-party condemnation of former ministers who were caught in an undercover "sting" operation for a television documentary.
Labour was forced by the revelations to rush forward a promise to enforce a compulsory register of lobbying which it said had been planned for the election manifesto.
But Mr Cameron said the claims raised wider questions about whether the MPs had broken sleaze rules and urged the Prime Minister to probe potential breaches within the Government itself.
"These are shocking allegations. I have been warning for some time that lobbying would be the next scandal to hit politics," the Tory leader said.
"First of all the House of Commons needs to conduct a thorough investigation into these (former) Labour ministers but also the Prime Minister would want to get to the bottom of the allegations 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 an 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.
But there were immediate demands by opposition parties and a trade union for an inquiry into a series of policy changes that Mr Byers, who called himself "a cab for hire", said he secured.
Among Mr Byers' boasts was that he had come to a secret deal with Transport Secretary Lord Adonis over the termination of a rail franchise contract and that Business Secretary Lord Mandelson had got regulations on food labelling amended after he intervened on behalf of a supermarket giant.
All parties firmly denied the claims but the Tories and Liberal Democrats said they would table a series of Parliamentary questions seeking clarification from ministers about the claims and whether there had been any breach of the Ministerial Code.
Mr Byers also faces being reported to the Commons sleaze watchdog but said he was "confident" any investigation would show he had not breached the code of conduct.
Ms Hewitt, who served as health secretary, said she "completely rejected" the suggestion that she helped obtain a key seat on a Government advisory group for a client paying her £3,000 a day.
The work under discussion would have been taken up after she stepped down at the imminent general election and was no longer subject to the MPs' code of conduct, she pointed out.
Mr Hoon was reported to have wanted a £3,000 a day fee for work that would allow him to turn his political knowledge and contacts "into something that frankly makes money".
"At no stage did I offer, nor would I attempt to, sell confidential or privileged information arising from my time in government," he said.
Of 20 politicians contacted by the programme makers, 15 agreed to meet and 10 were invited in for interviews - nine of those being secretly filmed, of which six feature in the documentary.
The others were Labour's Margaret Moran, Baroness Morgan and Tory MP Sir John Butterfill.
An influential Commons committee called more than a year ago for a compulsory detailed register of all lobbying activity overseen by a powerful watchdog.
In response, the industry said it would merge a number of trade bodies to oversee activities and the Government said it would give the voluntary arrangement a chance to prove itself.
But, with Westminster still reeling from the public backlash over the expenses scandal, Labour said the latest revelations now warranted firmer action.
The Tories said they would tighten the rules over ex-ministers taking jobs linked to their former roles and put in place a series of other measures to improve transparency.
But they came under fire from campaigners for refusing to match Labour's pledge of a compulsory register in favour of continued self-regulation.
Foreign Secretary David Miliband said he was "appalled" by today's revelations.
"There is absolutely no room for the sort of innuendo or promises that seem to have been floated in this case," he said.
MPs are barred from being paid to influence ministers or legislation - but former MPs are not subject to such restrictions.
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