David Cameron is to pledge to change the law to prevent the "disgusting sight" of MPs using parliamentary privilege in a bid to avoid prosecution for expenses abuses.
The Tory leader will also challenge Gordon Brown to withdraw the Labour whip from three MPs who have been charged with false accounting over their claims.
And he will accuse the Prime Minister of helping to create the culture at Westminster that led to the expenses scandal and the collapse of public confidence in politics.
The intervention, in a speech in central London, comes amid fury over speculation that the trio - David Chaytor, Elliot Morley and Jim Devine - will argue that parliamentary privilege exempts them from prosecution.
Home Secretary Alan Johnson has flatly rejected the idea that the principle should provide a "get out of jail card" for MPs.
"My colleagues in parliament should get a fair trial," he told the BBC's Andrew Marr show. "That fair trial should be on the same basis as any member of the public who goes through the courts system."
Mr Cameron will insist that Mr Brown has shown he is "just not capable" of dealing with the issues involved in reforming parliament.
"Even worse, it's been reported that the Labour Party's official solicitor is providing the three MPs with legal advice," he will say.
The Labour Party rejected the suggestion that it was providing legal advice for Mr Morley, Mr Chaytor and Mr Devine.
A Downing Street spokesman said: "The Director of Public Prosecutions has said its for the courts to determine whether parliamentary privilege applies. If necessary we will legislate to clarify this."
