LONDON (Reuters) - Prime Minister Theresa May has been left exposed at the top of a government unable to unite over Britain’s plans to leave the European Union, after Foreign Secretary Boris Johnson and Brexit Secretary David Davis quit within hours of each other.

Britain's Prime Minister Theresa May commences a meeting with her cabinet to discuss the government's Brexit plans at Chequers, the Prime Minister's official country residence, near Aylesbury, Britain, July 6, 2018. Joel Rouse/MOD/Handout via REUTERS

Below is an explanation of how May could be removed from office if she has to face a leadership challenge:

- What needs to happen for there to be a leadership contest?

A leadership challenge can be triggered if 15 percent of members of parliament in May’s Conservative Party write a letter to the chairman of the party’s so-called “1922 committee”.

The Conservatives currently have 316 members of parliament (MPs) so 48 of them would need to write such letters to challenge May.

Once that threshold has been reached, the chairman will announce the start of the contest and invite nominations.

- Could this happen to May?

The chairman of the 1922 committee is the only person who will know exactly how many members of parliament have submitted letters of no confidence.

But some eurosceptic members of parliament have started submitting letters to the committee chairman in protest over her Brexit negotiating strategy.

- What will happen during a no confidence vote?

If a no confidence vote is called then all serving Conservative members of parliament will be able to cast a vote for or against the serving leader.

If May wins any confidence vote she remains in office. If she loses, she is obliged to resign and barred from standing in the leadership election that follows.

- How quickly can a no confidence vote take place?

In the last no confidence vote against a sitting Conservative leader in 2003, the chairman of the 1922 committee announced he had received enough letters to trigger a vote on Oct. 28 and the vote was held the next day.

- What would happen if May lost the no confidence vote?

If May lost a no confidence vote then there would be a leadership contest.

If several names are put forward to lead the party, then a vote is held among Conservative MPs using the first past the post system to whittle down the field with the candidate with the fewest votes removed. Another ballot among Conservative lawmakers is then held until two candidates remain.

The final two nominees are then put to a ballot of the wider Conservative Party membership with the winner named the new leader.

Following David Cameron’s decision to step down as prime minister and Conservative leader after the EU referendum in 2016, five candidates put their names forward.

The field was narrowed to May and then junior minister Andrea Leadsom but she pulled out of the race before members voted, leaving May to become leader unopposed.