Fallon's shock resignation this evening has already prompted speculation about his replacement.

Penny Mordaunt MP - a defence minister between May 2015 and July 2016 - is one person tipped for the role.

BBC political editor Laura Kuenssberg said "lots more names will swirl around" ahead of the PM announcing a new defence secretary.

Jonathan Beale
BBC defence correspondent

Commenting on Sir Michael Fallon's resignation, a defence source told the BBC the Ministry of Defence had lost a powerful voice and strong advocate of the Armed Forces at a critical time, when they are facing "difficult decisions".

Sir Michael had recently called for the government to do more on defence spending.

The government is currently conducting a security and defence review that could lead to further cuts in the military as the department controls spending.

A source close to Sir Michael told the BBC that he took the decision to resign this afternoon.

Image source, PA

Sir Michael Fallon had an interrupted parliamentary career that spanned four decades and two constituencies.

In March 1983, he lost the Darlington by-election to Labour's Oswald O'Brien, only to win it 77 days later after Margaret Thatcher called a general election.

But in 1992 his career in government stalled after he lost his Darlington seat Labour's Alan Milburn in the General Election.

He returned to Westminster in 1997 after being selected as the Conservative candidate for Sevenoaks when MP Mark Wolfson retired.

During the coalition government he was appointed minister for business and enterprise, and then minister for energy.

He was then appointed minister for Portsmouth in 2014 by David Cameron - a post which was created after the loss of jobs in the local shipyard at arms manufacturer BAE Systems.

In the same year he succeeded Philip Hammond as defence secretary.

Labour seeks to use an "ancient" Parliamentary tool to force the government's hand.

BBC News Channel

BBC News Channel

Sir Michael Fallon expanded on his reasons for resigning in an interview with BBC political editor Laura Kuenssberg.

"I have behaved in the past, clearly, in a way that is below the standards that is required of the Armed Forces," he said.

He thought the culture in Westminster had "changed" over the years, explaining: "What might have been acceptable 15, 10 years ago is clearly not acceptable now."

He added: "I think we've all got to look back now at the past, there are always things you will regret, will have done differently."

In his resignation letter, Sir Michael Fallon said a number of allegations had surfaced about MPs in recent days, "including some about my previous conduct".

But he said many of these allegations had been false.

However, he added: "I accept that in the past I have fallen below the high standards that we require of the Armed Forces that I have the honour to represent."

Theresa May has spent the evening attending the Spectator's Parliamentarian of the Year awards.

Radio 4 Today presenter Nick Robinson describes the evening as the "Oscars of the political world".

BBC News Channel

Former head of the Army, General Sir Mike Jackson, said members of the Armed Forces will be "sad" to see Sir Michael Fallon go.

He told the BBC News channel: "It's clearly a personal decision he's come to, and so be it."

The resignation comes a day after a spokesman for Sir Michael confirmed he was once rebuked by journalist, Julia Hartley-Brewer, for putting his hand on her knee during dinner in 2002.

House of Commons
Parliament

Image source, hoc

A Labour motion requiring the government to release assessments of the impact of Brexit has been passed by MPs.

Speaker John Bercow told MPs he could consider contempt of Parliament claims if the government fails to release Brexit impact assessments.

Labour's motion which asked for a "humble address" requesting the Queen to direct Brexit Secretary David Davis to release the documents,

There was confusion during the debate about whether a vote triggered by Labour's use of an arcane parliamentary procedure would be binding.

Mr Bercow said motions of this kind have "traditionally been regarded as binding or effective".

The BBC has been told that the Defence Secretary Sir Michael Fallon has resigned following allegations of past behaviour.

House of Commons
Parliament

The Commons meets tomorrow from 9:30am for question to Brexit Secretary David Davis.

Expect a continuation of the themes of today's debate, at least in topical questions.

MPs will also debate backbench motions on unaccompanied child refugees in Europe, and sexual harassment and violence in schools.

House of Commons
Parliament

After a debate on parliamentary procedure that has tested our knowledge, it's time for the final, short adjournment debate.

Labour MP Chi Onwurah has secure parliamentary time to discuss Newcastle charity the St Mary Magdalene and Holy Jesus Trust.