Gordon Brown today suspended ex-minister Elliot Morley from the Parliamentary Labour Party and an aide to Tory leader David Cameron quit his post, as the Westminster expenses scandal claimed its first scalps.
Mr Morley, MP for Scunthorpe, now faces expulsion from Labour ranks if charges that he wrongly claimed £16,000 in allowances for a mortgage already paid off are backed up by Parliamentary Standards Commissioner John Lyon.
The Prime Minister also suspended former fisheries minister Mr Morley from his post as the premier's climate change envoy.
Bracknell Conservative MP Andrew MacKay announced his resignation as a parliamentary aide to Mr Cameron after confessing he had made an "error of judgment" on his second homes claims.
Mr MacKay, who is married to Bromsgrove MP Julie Kirkbride, claimed the full second-home allowance for interest on their joint mortgage for a London home until April this year.
At the same time his wife also claimed the full annual sum for mortgage interest on a constituency home.
Labour chief whip Nick Brown said Mr Morley - who referred his own expenses today to Westminster's sleaze watchdog - had been suspended from the PLP "pending further investigation by the Parliamentary Commissioner on Standards and without any presumption of guilt".
But few at Westminster doubt the MP will be cut adrift from Labour unless compelling evidence to explain his error of wrongly claiming thousands of pounds over 18 months is found.
The Prime Minister, speaking at what should have been a flagship launch of his party's MEP and council election campaign, said: "Where standards are transgressed and mistakes are made, we have got to take action.
"That is why today we have suspended from the Parliamentary Labour Party Elliot Morley because of the allegations, which are serious, which have been made against him."
The premier added: "Where disciplinary action is necessary it will, and will immediately, be taken.
"If there are any other disciplinary cases where we have to take action, we will take action immediately.
"But I think what people want to see is a system that is reformed, politics seen to be serving the public interest, people recognising that as a country we depend on trust in our political system and trust in the people who actually are in offices of authority and influence, and they have a duty to show that that trust is deserved."
Mr Morley said he had already paid back the £16,000 he had wrongly claimed and added: "I deeply apologise for such sloppy accounting in a very loose and shambolic allowance system but there is nobody to blame but myself and I take full responsibility for this."
Nick Brown today also hauled in MP Fabian Hamilton (Leeds North East), who claimed thousands of pounds in second home allowances for his family home in Leeds while listing his mother's property in London as his main residence.
The Chief Whip said that he would work on the basis of "check the facts first and then decide how to act".
But he told Sky News: "The interests of the Labour Party are very clear. We come into public service to do the right thing by the people who elect us to represent them.
"If there has been wrong-doing it has to be put right as quickly as possible and if it is very serious there will be a very serious response."
Speaking on the campaign trail in Cornwall, Mr Cameron said: "We know how angry people are. We know how disappointed they are. We are going to have to rebuild confidence in parliament bit by bit and brick by brick."
Earlier, he described Mr MacKay's allowance arrangements as "unacceptable".