Gordon Brown says there are clear cases of MPs who may have broken the law over expense claims but insisted that only a few MPs had abused the Westminster perks system.
The Prime Minister said he planned to introduce a binding code of conduct for MPS and warned other public institutions they would also face tougher scrutiny as part of a bid to clean up the system.
And he hinted that an independent review of the pay and perks system would call for a ban on controversial golden handshake pay-offs for MPs who stand down.
Speaking amid further embarrassing revelations of expense claims, including one Labour MP's bid to get back a £5 church donation from the taxpayer, the PM expressed his shock.
What I have seen offends my Presbyterian conscience, what I have seen is something that is appalling, he told BBC1's Andrew Marr Show.
I did not expect to see instances where there are clear cases which maybe have to be answered to for fraud.
Most MPs are doing a good job. They are in for public service, they are in for what they give not what they can get. But where a few MPs have abused the system they have got to pay back and there has got to be punishment and discipline.
Asked if he wanted Scotland Yard, which is presently considering a criminal investigation, to get involved, he said: That is a matter for the police.
Setting out plans for a Constitutional Reform Bill, he said it would include ”a clause which sets out the responsibilities, a code of conduct for MPs.