London - Members of British Prime Minister Theresa May's Conservative party have triggered a vote of confidence in her leadership. If she loses, her successor would take over in Downing Street. If she wins, she will be immune from further challenge for a year.

Here is how the process works:

- 48 letters -

A vote of confidence can only take place when requested by 15 percent of the Conservative parliamentary party -- currently 48 MPs.

They must submit letters to the MP who chairs the 1922 Committee of Conservative backbenchers, that is lawmakers who hold no ministerial office and are well-known for taking a more independent and critical line on the issues of the day.

The current chairman, Graham Brady, announced on Wednesday morning that the threshold has been reached and called a ballot for Wednesday evening.

Profile of British Prime Minister Theresa May

- Confidence vote -

Conservative MPs will decide the prime minister's fate by secret ballot, scheduled for between 6:00pm (1800 GMT) and 8:00pm.

"The votes will be counted immediately afterwards and an announcement will be made as soon as possible," Brady announced in a statement.

If May wins -- for which she needs a minimum 159 Conservative MPs to back her -- she cannot be challenged again for a year.

However, if her victory is only by a small margin her authority may be fatally undermined.

If she loses, a leadership contest follows in which she cannot stand.

- Leadership contest -

Candidates must be Conservative MPs and have the support of at least two colleagues.

Assuming there are several, Conservative MPs then vote in a series of secret ballots, eliminating the least popular candidate until two nominees remain.

The final pair are then put to a vote of all Conservative Party members, with debates between candidates held across Britain over several weeks.

The process can go more quickly -- May won the leadership in July 2016 without a vote by party members after her rival, Andrea Leadsom, pulled out.

The composition of the UK parliament

- Why now? -

Hardline eurosceptic MPs have long claimed to have the numbers to trigger a vote of no confidence, but have held off because they believed May would win.

An attempt to launch a challenge failed last month after the prime minister struck a Brexit deal with the European Union, despite widespread criticism of the agreement.

But the situation changed after May pulled a parliamentary vote on the deal on Monday amid fears she would lose by a significant margin.

A new Conservative party leader, who automatically becomes premier, would still face the difficult circumstances that she has faced in dealing with Britain's withdrawal from the European Union, scheduled for March 29.

The Conservatives do not have a majority in the House of Commons, nor is there a majority of MPs from any political party for one vision of Brexit.

The parliamentary arithmetic will not change without a snap election, something many Conservatives oppose because of the fear of losing office to Labour.

By Alice Ritchie