Michael Martin is understood to be preparing to announce he will stand down as Commons Speaker.

The announcement is set to be made in the House of Commons later, the BBC has reported.

Mr Martin has been criticised over his handling of the furore over MPs' expenses and a motion of no confidence in him has been backed by 23 MPs.

It is not known if he will stay until the election or go immediately. It is the first time in 300 years a Speaker has been effectively forced out.

Yesterday he said he was "profoundly sorry" for his part in events but did not give any indication - or timescale for any plans to stand down.

He was challenged by a succession of MPs who stood up to ask when they could debate a motion of no confidence in him.

Liberal Democrat leader Nick Clegg had openly called for him to go - calling him a "dogged defender of the status quo". Prime Minister Gordon Brown and Tory leader David Cameron had said it was a matter for the House of Commons - not the government or official opposition.

Mr Martin, the MP for Glasgow North East and Speaker since 2000, is due to meet party leaders later to discuss interim reform proposals for MPs' expenses.