LONDON (Reuters) - Lawmakers in British Prime Minister Theresa May’s Conservative Party on Wednesday triggered a confidence vote in her leadership over Britain’s planned divorce from the European.

Britain's Prime Minister Theresa May returns to Downing Street in London, Britain, December 10, 2018. REUTERS/Toby Melville

Below is an explanation of how the leadership challenge works under the ruling Conservative Party’s rules.

- HOW WAS IT TRIGGERED?

A challenge was triggered after 15 percent of the Conservative members of parliament (MPs) wrote 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 were needed to write such letters for the vote to be called.

- WHEN WILL IT HAPPEN?

Graham Brady, the chairman of the 1922 Committee, said a ballot would be held between 1800 GMT and 2000 on Wednesday, Dec. 12, in committee room 14 of the House of Commons.

He said he expected lawmakers to select one of two neutral propositions: either - I have confidence in Theresa May as leader of the Conservative Party, or, I do not have confidence in Theresa May as leader of the Conservative Party.

The votes will be placed into a metal ballot box and then counted by Brady and his two vice chairs. The count will be verified and then announced on Wednesday evening.

- 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, but Brady said he still needed to see the definitive list as two lawmakers may not be allowed to vote because they have been suspended from the party.

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. However, a significant mutiny could make her leadership untenable.

- 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 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.