The Commons Speaker is to meet party leaders to discuss expenses reforms, as pressure mounts for him to quit.
The Speaker is meeting party leaders to discuss expenses reforms, shortly after announcing his resignation.
On Monday Michael Martin apologised for his role in events but a succession of MPs stood up to tell him to go.
Michael Martin, who will stand down on 21 June, has already urged MPs to stop making claims until new interim rules is in place.
He is blamed by some for efforts to keep expenses secret but others say he is being made a scapegoat.
Prime Minister Gordon Brown said earlier he would be proposing a "fundamental rethink" of the system.
A no confidence motion has been signed by 23 MPs. Meanwhile Labour chiefs are to discuss the future of two Labour MPs suspended over their expenses claims.
It follows damaging revelations in the Daily Telegraph based on leaked receipts of MPs' claims.
The party's ruling National Executive Committee will meet to discuss what should be done about any of its MPs who have broken the rules.
In a brief statement to a packed Commons chamber at 1430 BST, Mr Martin said he would step down as Speaker to maintain "unity" in the House.
It followed unprecedented scenes in the Commons on Monday when a succession of MPs stood up to challenge him - he has been criticised for his handling of the expenses scandal and accused of defending the status quo.
It is understood that it will focus on two MPs suspended from the party - former minister Elliot Morley and David Chaytor - for claiming for mortgage interest payments when their mortgages had been paid off.
The motion of no confidence in the Speaker - MP for Glasgow North East - has been formally placed on Tuesday's Commons order paper.
It has been tabled by Conservative backbencher Douglas Carswell, but former Tory minister Peter Bottomley has tabled an amendment, saying that the Speaker himself is "best placed to decide when to retire from office".
The meeting organised by Mr Martin for all party leaders is intended to thrash out immediate proposals to reform the allowances system - ahead of the findings of an independent review, due in the autumn.
The meeting organised by Mr Martin for all party leaders is intended to thrash out immediate proposals to reform the allowances system - ahead of the findings of an independent review, due in the autumn.
Mr Martin told MPs on Monday: "While we await the outcome of his work, it is imperative that we continue to improve our accounts and practice in the interim and get in place measures that work and are seen to be working."
Threshold cut
But a meeting of party leaders on the same issue several weeks ago failed to reach agreement.
But a meeting of party leaders on the same issue several weeks ago failed to reach agreement.
Conservative leader David Cameron has repeatedly refused to call for the Speaker to go, saying it would not be right for the main opposition party to do so.
Mr Brown's announcement in a video on the Downing Street website and YouTube that he wanted to scrap the second homes allowances and replace them with a daily attendance allowance instead was widely criticised.
He told BBC Radio 4's Today programme: "The Speaker has got to be above party politics. I will go to that meeting to discuss what we can do to clean up MPs' allowances."
He, Conservative leader David Cameron and Lib Dem leader Nick Clegg failed to reach an agreement and he had to drop the proposals the following week.
'Difficult questions'
However MPs did vote through some changes to the second homes allowance - including stopping MPs in greater London from claiming it and reducing the threshold for claims which required receipts from £25 to zero.
But he said the no confidence motion should be debated, if signed by enough MPs.
Earlier Mr Brown said he would propose "a fundamental rethink of the system, a root and branch reform" and said changes would happen "very soon".
"Clearly the Speaker is in a difficult position, he has to answer a series of very difficult questions," he added.
"I will be proposing major changes in the way the whole system operates. And I believe the system that will emerge over the next few weeks is quite different from what we've had."
The BBC's political editor Nick Robinson said the government controlled whether the no confidence motion would be given time for debate.
Mr Brown is expected to give a news conference at 1730 BST.
DEFENDING INDEFENSIBLE? What about claims for televisions which would obviously be used for watching all sorts of programmes? Such items passed the test of being allowable as they were needed "for political purposes" such as watching the news, he said. How expenses chief defended system
Liberal Democrat leader Nick Clegg - who has openly called for Mr Martin to go and has said he is "minded" to sign the no confidence motion himself - will also be at the meeting.
He told the BBC if the government would not allow the debate, he was "already looking at ways I can call that debate and the vote".
Prime Minister Gordon Brown declined to give Mr Martin his backing, saying that "the decision about who is Speaker is a matter for the House of Commons - it could never be a matter for the government".
In a statement to the Commons on the expenses furore on Monday, Mr Martin did not mention his future - instead he set out proposed action to reform the system.
In a statement aimed at the public, he told MPs "We must all accept the blame and, to the extent that I have contributed to the situation, I am profoundly sorry. Now each and every member, including myself, must work hard to regain your trust."
1695
He also urged MPs not to file expenses claims until a new set of rules are agreed upon for the interim period before Sir Christopher Kelly's independent inquiry reports back.
But a succession of MPs challenged him openly, saying they wanted a debate and no confidence vote.
David Cameron: "Speaker has to be above politics"
It follows more than a week of damaging newspaper revelations about MPs' expenses and criticism of the way Mr Martin has handled the affair.
His critics blame him for various attempts to block requests in recent years to have expenses details released under Freedom of Information laws.
And he angered many by attacking MPs who had pressed for more transparency or criticised his decision to ask the police to investigate the leaking of expenses details to the Daily Telegraph.
For MPs to openly criticise the Speaker breaks a long-standing Commons convention and the last time a Speaker was forced from office was in 1695 - when Sir John Trevor was found guilty by the House of "a high crime and misdemeanour".
