David Cameron stepped up demands today for a "brief but comprehensive" inquiry into the Government lobbying row.
The Tory leader insisted the case for a full investigation was "incredibly strong" after three former Cabinet ministers were suspended from the Parliamentary Labour Party last night.
Gordon Brown has dismissed the need for an inquiry after receiving assurances from Whitehall department heads that no improper influence was exerted.
But Mr Cameron said ministers were saying different things about their discussions with Stephen Byers, who was secretly filmed claiming he had changed Government policy.
"That's why we need a proper inquiry into all this," Mr Cameron told a regular Westminster press conference.
"We do know that the policies referred to did actually change, so we need to see the minutes of meetings, the emails, the telephone logs, those things, to rapidly establish what did actually happen.
"If it was serious enough to strip these former ministers of the party whip, it is surely serious enough for a brief but comprehensive inquiry.
"But Gordon Brown has decided to rule out an inquiry even before the Labour whips had seen the programme and decided to take the action against their MPs.
"I believe he needs to think again."
Ministers were today seeking to distance the Government from the row which has engulfed Mr Byers, Geoff Hoon and Patricia Hewitt, who are all facing allegations.
The claims emerged in a Dispatches programme, aired by Channel 4 last night, in which they were interviewed by what they thought was an American lobbying firm.
Justice Secretary Jack Straw said there was "anger" and "incredulity" among the MPs' Labour colleagues.
But Mr Cameron said the row would further damage the public's faith in politicians in general, coming on the heels of the expenses scandal.
"Our political system is looking evermore ragged and broken," he said.
"Why? Because it depends on public trust and that trust has not just been damaged, it is in danger of disappearing altogether.
"Every time we patch up one part of the broken politics another crack appears elsewhere."
However, another MP featured in last night's Dispatches programme was a Tory backbencher, John Butterfill.
He has referred himself to the Parliamentary Commissioner for Standards, John Lyon.
Former Labour minister Baroness Morgan of Huyton, also facing claims, said she was referring herself to the House of Lords' Sub-committee on Lords' Interests.
Backbench Labour MP Margaret Moran was also suspended after featuring in the programme.
Mr Straw said she and the three former Cabinet ministers had been suspended under a Labour standing order against bringing the party into disrepute.
"It's my view certainly, having seen what I have seen, that their behaviour, prima facie, does indeed bring the Parliamentary Labour Party, as well as Parliament, into disrepute, because it appears that former Cabinet ministers are more interested in making money than they are in properly representing their constituents," he told the BBC.
"That's why there is such anger in the Parliamentary Labour Party, as well as I may say incredulity, about their stupidity in allowing themselves to be suckered in a sting like this."
Mr Straw said an investigation into any potential impropriety by ministers or officials - as called for by the Tories - had been carried out.
"There is not a shred of evidence - not a single scintilla of evidence - of any impropriety whatsoever, that's why it's been swift," he said.
Whitehall chiefs have looked into claims raised in the documentary and told Cabinet Secretary Sir Gus O'Donnell they are satisfied there has been "no improper influence".
In a letter to shadow Cabinet Office minister Francis Maude, Sir Gus said that he and Mr Brown had taken steps to ensure the claims that serving ministers and officials had been lobbied were "immediately investigated".
"Permanent secretaries in the relevant departments have looked in to these issues as a matter of urgency, as they would with any such serious allegations," he said.
"The permanent secretaries have assured themselves and advised the Prime Minister and me that there was no impropriety by current ministers or officials."
Mr Cameron said today that Sir John Butterfill will not get a place in the House of Lords following the revelations on last night's programme.
The long-serving Bournemouth West MP, who is standing down at the election, was filmed telling the undercover reporter that he would be a useful contact to lobby a future Conservative administration, as he was "quite likely" to be made a peer.
Mr Cameron said the programme featured "a Conservative backbencher who attempted to up his value by suggesting he will get a peerage", adding: "I can tell you that's not going to happen."
The Tory leader brushed aside suggestions that he should follow Labour by withdrawing the whip from Sir John.
"The right step to take is for him to refer himself to the Parliamentary Standards Commissioner, which has happened," he said.
"If he has done anything wrong, the whip can be withdrawn, but withdrawing the whip from someone two weeks before the election is not the most heinous punishment."
Mr Cameron acknowledged that the last Tory government had ended with similar scandals, and predicted that an incoming administration would be a target for lobbyists.
"I'm sure that if we win the general election corporate interests, corporate money, lobbying will be all over new Members of Parliament, new ministers, the new government, like a rash," he said.
"That's why I want to do something about it and do something about it now. We need to act."
He promised that a Tory government would not allow former ministers to use their government knowledge and contacts for private gain, and would:
* Double the time it is forbidden for ex-ministers to lobby the Government, from 12 months to two years;
* Extend to 10 years the period during which ex-ministers must seek advice from the Advisory Committee on Business Appointments, as well as putting the committee on a statutory basis so that its advice was binding in law;
* Instruct the Prime Minister's adviser on the ministerial code to undertake a full review of the current row, identifying any necessary changes in the rules to avoid a repeat;
* Cancel Government agencies' contracts with lobbying firms.
Downing Street today again rejected Mr Cameron's demands for an inquiry, saying that the matter had been dealt with in Sir Gus's letter to Mr Maude.
Asked how they could be confident that there had been no impropriety, the Prime Minister's spokesman said: "We have very good records."
The spokesman said that Mr Brown condemned the alleged actions of the three former ministers.
"The Prime Minister is clear that there must be no blurring of the line between public service and private gain," he said.
Broadcaster Esther Rantzen, who is standing as an independent Ms Moran's Luton South seat in the general election, said: "It is not enough for her to be suspended from the Parliamentary Labour Party - she should have been suspended from Parliament.
"Luton South has been without representation for a year and it is quite appalling after the fact she said she was unavailable for work which might improve the lives of her constituents."
Business Secretary Lord Mandelson dismissed Mr Cameron's calls for an inquiry and he attacked the Tory leader for not taking action against Sir John.
"I don't really think it befits him to try to make party political capital out of the situation when he, I notice, has not suspended the Tory MP who was featured in last night's film," he told the BBC.
"The best remark I can make about Mr Cameron is that people in glass houses should not throw stones."
Labour leader of Ashfield District Council John Knight, who is a member of Mr Hoon's constituency party, said he was "disgusted" to see the MP caught up in the Dispatches investigation.
Mr Knight said the Ashfield MP was typical of a generation of "careerists" in the Parliamentary Labour Party, and warned Mr Brown that if he did not tackle the issue, the party was heading for a "slow death".
"I would like to tell you I was shocked and surprised, but frankly I wasn't," Mr Knight told BBC Radio 4's The World At One.
"I would have been more shocked and surprised if he had said 'I don't want this money, I'm already a millionaire, I want to devote the rest of my political career, such as it will be, to the working-class people of Ashfield'. That would have made me fall off my seat.
"I was disgusted really.
"The real tragedy is it is not just about Geoff. He is a man who personifies an entire generation of middle-class Labour MPs who have absolutely no affinity or understanding of the people who they are supposed to represent.
"This is a man who really didn't care about the ordinary working-class people in Ashfield - he was out for his career."
Mr Knight added: "Soon the entire Parliamentary Labour Party will be absolutely filled to the gunwales with this kind of person. It really needs review and reform from within.
"I am really looking to the Prime Minister to sort this out, because really if we carry on like this, this will be the slow death of the Labour Party. It will be a black hole that implodes on itself."
Commenting on the Dispatches report, the chairman of the Committee on Standards in Public Life (CSPL), Sir Christopher Kelly, told The World At One: "I am greatly saddened by what I saw and by the further damage it will do to people's perceptions of MPs."
Sir Christopher said he was not in a position to judge whether the internal departmental checks - on the basis of which Mr Brown said yesterday he was "satisfied" there had been no impropriety by serving ministers - had been adequate.
It was for Cabinet Secretary Sir Gus O'Donnell to answer for them, he said.
The CSPL head, who drew up proposals for reform of MPs' expenses last year, said there was no need for new rules on lobbying by parliamentarians.
"What we don't want is a new set of rules in response to every scandal," said Sir Christopher. "That way lies madness and excessive bureaucratisation.
"What we need is clear rules - and we have those in relation to MPs. It is quite clear that paid advocacy is banned.
"It requires proper enforcement of those rules and adequate sanction when the rules are broken. What is most needed is a change of behaviour so the principles of public life are embedded in those in public office."
Sir Christopher renewed calls for independent members to be appointed to the Commons Standards and Privileges Committee, which rules on allegations of misbehaviour by MPs but is entirely made up of MPs.
He said falling levels of trust in MPs were "deeply damaging", adding: "Our democracy does depend on people having confidence in those legislating for them."
Conservatives said that they would ban Government quangos from hiring lobbyists, as many currently do.
They released details obtained from parliamentary questions showing that more than 50 quangos are using, or have recently used, public money to contract public relations companies and lobbyists.
Shadow Cabinet Office minister Nick Hurd said: "This is systematic of a corrosive culture of spin in Whitehall and the unhealthy influence of lobbyists in Labour's corridors of power.
"Conservatives will adopt US-style rules to ban these practices and we challenge Labour ministers today to serve notice on every lobbying firm and cancel their contracts.
"It is a scandalous waste of taxpayers' money for Government bodies to be hiring lobbyists to lobby the Government and try to pressure Conservatives from holding the Government to account. No wonder the bureaucratic state has ballooned under Labour, given lobbyists are being bankrolled using public funds to justify yet more state spending and regulation."