David Cameron has stepped up demands 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 on Monday night.
Gordon Brown has dismissed the need for an inquiry after receiving assurances from Whitehall department heads that no improper influence was exerted.
Ministers are seeking to distance the Government from the row which has engulfed Stephen Byers, Geoff Hoon and Patricia Hewitt, who are all facing allegations.
The claims emerged in a Dispatches programme, aired by Channel 4 this week, 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 Monday night's Dispatches programme was a Tory backbencher, John Butterfill. He has referred himself to the Parliamentary Commissioner for Standards.
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.
