LONDON (Reuters) – Prime Minister Theresa May’s position as leader of the Conservative Party – and consequently her premiership – is under threat from lawmakers who are unhappy at the Brexit deal she has agreed with the European Union.

The BBC reported on Tuesday, citing multiple sources, that a leadership challenge had been triggered, although there was no official confirmation of this.

Below is an explanation of how May could face a leadership challenge under the ruling Conservative Party’s rules:

– WHAT NEEDS TO HAPPEN FOR THERE TO BE A LEADERSHIP CONTEST?

A challenge is triggered if 15 percent of the Conservative members of parliament (MPs) write letters demanding a confidence vote to the chairman of the party’s “1922 Committee”, which represents those lawmakers who have no government jobs.

The Conservatives have 315 MPs, so 48 would need to write such letters for a vote to be called.

– COULD THIS HAPPEN TO MAY?

In recent months, some eurosceptic MPs have publicly said they have submitted such letters in protest at her Brexit strategy. The number voicing concern rose after she delayed a planned vote on her Brexit deal in parliament when facing an almost certain defeat.

However, the 1922 Committee’s chairman, Graham Brady, is the only person who knows how many have actually submitted letters, including those who have written to him confidentially.

Brady has not commented on the latest media reports that the threshold has been breached.

– WHAT HAPPENS DURING A CONFIDENCE VOTE?

All Conservative MPs can vote. May would need a simple majority of the total votes registered in order to win. If all elected lawmakers cast their ballots, that would currently mean 158 votes.

If May wins, she remains in office and cannot be challenged again for 12 months. If she loses, she must resign and is barred from standing in the leadership election that follows.

– HOW QUICKLY CAN A VOTE TAKE PLACE?

Under Conservative rules, the vote is held as soon as possible, on a date decided by the 1922 Committee chairman in consultation with the party leader.

The last no-confidence vote against a Conservative leader, when the party was in opposition in 2003, was held the day after the Committee chairman announced he had received enough letters.

– WHAT HAPPENS IF MAY LOSES?

There will be a leadership contest to decide her replacement. Her replacement will become prime minister, but a general election will not automatically be triggered.

If several candidates come forward, a secret vote is held among Conservative MPs to whittle down the field. The candidate with the fewest votes is removed and Conservative lawmakers vote again. The process is repeated, on Tuesdays and Thursdays, until only two candidates remain.

These two candidates are then put to a postal ballot of the wider Conservative Party membership. Participants need to have been party members for more than three months.

When David Cameron decided to step down as prime minister and Conservative leader after the EU referendum in 2016, five candidates came forward. The field was narrowed to May and then-junior minister Andrea Leadsom, but she pulled out before members voted, leaving May to become leader unopposed.

– WHO COULD REPLACE MAY?

The list of possible replacements is long, but there is no clear frontrunner. Here is a summary of some potential candidates: