Michael Martin will stand down as Speaker of the House of Commons within hours, STV news understands, a historic moment in British politics that will end a difficult period for the Glasgow MP amid a wave of criticism from Westminster.
As reported first by STV news, the Speaker will inform ministers of his plan to leave his position in a 2.30pm announcement.
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LIVE COVERAGE - STV NEWS
Watch full coverage of Michael Martin's resignation at 2.30pm >>
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Overnight Mr Martin reflected on his position amid a growing rift over handling of expense claims reported by MPs. The Labour MP then spoke with Prime Minister Gordon Brown on Tuesday morning, STV news has learned.
David Cameron joined the chorus of MPs who want to see Mr Martin leave his position. The Conservative Party leader offered his no confidence in the Speaker on Tuesday, hours after hearing similar voices of discontent from fellow ministers.
Mr Martin will become the first Speaker to be forced from his post in more than 300 years. Sir John Trevor was ousted from the office in 1695 after the House of Commons found him guilty of "a high crime and misdemeanour" for accepting a bribe.
The political future for the MP for Glasgow North East now hangs in the balance and a by-election could follow.
STV news political editor Bernard Ponsonby broke the story at 10.15am on Tuesday.
He explained the complexity of Mr Martin's decision to stand down, saying: "This is an awkward position for the House and indeed the Prime Minister to be in. And it opens up the possibility of a squabble between the two main parties.
"The last two Commons Speakers have come from Labour. Many in the house will want a Speaker from outwith the governing party but that could be vetoed by Labour MPs if the succession becomes a matter of party politics.
"Also complicating this is the Speaker's position as a constituency MP. Michael Martin was elected as 'The Speaker'. Should his resignation from this position lead to a by election in his Glasgow North East constituency? That could become clearer later in the day."
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RESOURCES
Video: Michael's Martin's full statement to the House of Commons (May 18)
Video: Michael Martin 'Seven Days' interview from 2000
Michael Martin: From shop steward to Speaker
The Speaker - a historical perspective
Political reaction to the Speaker's decision
Secret ballot to select next Speaker
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A spokeswoman for the Speaker said: "I can confirm that the Speaker is making a statement this afternoon and that it is about himself."
She said Mr Martin would still be going ahead with his meeting with the party leaders at 4.30pm to discuss reform of the system of MPs' expenses.
He had summoned Mr Brown and other party leaders for the talks as politicians grappled to contain the crisis that has erupted following the disclosure of details of MPs' second home claims.
The Prime Minister's spokesman said a morning Cabinet meeting on Tuesday had discussed proposals on the reform of parliament and the regime governing MPs' expenses. Mr Brown told ministers about a package of proposals he will put to the meeting with the Speaker later in the day.
His spokesman refused to speculate on the Speaker's future, saying: "That's a matter for the Speaker."
STV news understand that Mr Martin spent Tuesday morning informing his closest supporters of his decision to resign as Speaker.
STV is endeavouring to determine whether Mr Martin will resign from the Commons and force a by-election in Glasgow North East, a decision the Labour Prime Minister would rather not make. Labour have a large majority in the House of Commons but a third consecutive Labour Speakers could prompt a row.
The Speaker's future was left in the balance following extraordinary scenes in the Commons on Monday when a series of MPs rose to demand he stand aside or submit to a vote of no confidence.
Despite saying he was "profoundly sorry" for his part in what had happened, he showed no indication that he was prepared to go.
The UK Government had resisted pressure to allow a Commons debate on a motion of no confidence in Mr Martin tabled by the Tory backbencher Douglas Carswell. That motion had been backed by 23 MPs by this morning.
Faces of Frustration: Michael Martin in a challenging day as Speaker
Speaking in Westminster on Monday, he said all members bear responsibility for what had happened and they all must work to restore the trust of the voters. He added: "We all bear a heavy responsibility for the terrible damage to the reputation of this House. We must do everything we possibly can to regain the trust and confidence of the people."
He urged all MPs not to submit expenses claims for approval until interim reforms had been agreed and said an early day motion calling for him to be replaced was not "substantive". Mr Martin also refused to say whether he would be stepping down before the next general election.
His statement did little to quell anger among MPs. Conservative MP Richard Shepherd urged that the motion of no confidence in Mr Martin should be debated, but the Speaker said that was a matter for the UK Government.
Liberal Democrat leader Nick Clegg was the first party leader to add his voice to those calling for him to go, saying Mr Martin had "dragged his feet" in reforming the system.
Mr Martin, MP for Glasgow North East, angered critics by trying to block publication of MPs expenses and failing to push reforms of the system. He came under renewed fire last week for rebuking MPs who questioned his approach.
Previously, he had faced criticism over his family's claims for allowances, as well as facing high-profile questions over why police were allowed to search Conservative MP Damian Green's office without a warrant.
