Lawyers working for the Tax-Payers' Alliance pressure group want to question MPs suspected of committing crimes over their expenses to see if they have broken the criminal law.
They will invite politicians to make themselves available for interviews to find whether there is evidence to support a prosecution for fraud. Those asked to answer inquiries will be 'relevant witnesses, including the potential accused themselves'.
The move is the first step towards a private prosecution of MPs who might escape any investigation at the hands of the official criminal justice system.
The pressure group has launched a 'Bring Them to Justice' campaign aimed at securing fraud convictions against MPs whose behaviour over their expenses appears to amount to crime.
But senior police officers have given the alliance no assurances that Scotland Yard will mount a criminal prosecution or any indication that detectives have begun to secure documents or interview witnesses.
Police and the Crown Prosecution Service have set up a panel of officers and lawyers to assess the 'alleged misuse' by MPs of their expenses system.
However Acting Commander Nigel Mawer, the officer in charge of the panel, has declined to confirm whether it has ordered a criminal investigation.
There are fears that police will drag their feet because of high-level reluctance to become involved in another political scandal.
Senior officers are still bruised from the failure of the cash-for-honours inquiry and the fiasco of the arrest of Tory frontbencher Damian Green.
Among those MPs whose claims are under examination are Labour's Elliot Morley and David Chaytor, and Tory husband and wife Andrew MacKay and Julie Kirkbride.
Cabinet ministers under scrutiny include Home Secretary Jacqui Smith and Communities Secretary Hazel Blears.
Matthew Elliott, chief executive of the alliance, said: 'While we don't want to usurp the police inquiry, it is very concerning that the Met have not confirmed they will be looking into guilty MPs.
'It is most important that politicians are not above the law, and if there has been wrongdoing that they are subject to a full investigation just like the rest of us would be.