The resignations of Boris Johnson and David Davis over Theresa May’s Brexit plans have fuelled fevered speculation that the Prime Minister could face a leadership challenge. Here are some key questions answered:

– How would rivals launch a leadership challenge?

To trigger a no-confidence vote in the PM, 15% of Tory MPs must write to the chairman of the backbench 1922 Committee, currently Sir Graham Brady.

With 316 Conservative MPs in the House of Commons, Sir Graham must receive 48 letters to call a ballot.

– Are there enough?

According to reports, Sir Graham told a meeting on Monday night that he had not received the 48 letters required.

There are believed to be around 60 backbenchers in the Eurosceptic European Research Group (ERG), along with many others who would like to see a “harder” Brexit than the version set out at Chequers last week, making Mrs May vulnerable to an anti-EU revolt.

The ERG’s chairman, Jacob Rees-Mogg, has said he has not sent a letter to the 1922 Committee, and expects Mrs May to remain in office at least until Brexit Day in March 2019. Others may take their lead from him.

South Wales Argus:

(ERG chairman Jacob Rees-Mogg said he expects Theresa May to remain PM at least until Brexit Day in March 2019 (Jonathan Brady/PA))

– Who might take on the Prime Minister?

Mr Johnson and Mr Davis could be the front-runners in the event of a no-confidence vote, although other figures may launch bids of their own.

In his resignation letter, Mr Johnson did not back Mrs May to stay on as Prime Minister, while Mr Davis said she should.

According to the Daily Mail, Mr Rees-Mogg said on Monday night that Mr Johnson would make an “brilliant” prime minister.

– What if Mrs May refuses to stand aside?

If she chose to fight, she would need the support of more than 50% of Conservative MPs – currently 159 – in the confidence vote to stay in office.

But even if she achieved that threshold, a narrow victory would seriously undermine her authority and may lead her to question whether it was worth carrying on.

If she lost the vote, she would not be able to stand in the subsequent leadership contest, arranged by the chairman of the ’22.

– Why would critics not want to challenge Mrs May?

There are a number of issues that may make Eurosceptic critics hold back from an attempt to unseat the PM.

Aside from the loyalty which MPs naturally feel towards their leader, many are concerned that Mrs May’s removal could plunge the party into chaos, with no obvious replacement lined up, potentially setting the scene for Jeremy Corbyn to seize power in a new general election.

Some Brexiteers think the most crucial issue is to ensure that Britain actually leaves the EU in March next year, and feel that whatever arrangements Mrs May has secured can always be renegotiated once that point has been reached.

– What has she said?

Mrs May raised the prospect of a Jeremy Corbyn-led government to appeal for Tory unity on Brexit at a meeting of the ’22 on Monday night.

She said the alternative to the party coming together could be a left-wing Labour administration.