Prime Minister Theresa May is fighting for her political life.

As she prepares for the vote later today which will decide whether she stays in 10 Downing Street, it will not be Labour Leader Jeremy Corbyn and her Labour Party opposition who will decide her immediate fate. Instead it is her own Conservative Party MPs.

The magic number of 48 letters to the Chairman of the 1922 Committee has been reached, triggering a vote of no confidence in the Prime Minister.

For those unfamiliar with the inner workings of the Conservative Party, the process of challenging Theresa May's position as party leader may be confusing.

The Conservative Party has strict rules for choosing its leader.

Unlike the Labour Party, where members have a say from the first stage of the process, it is the MPs who decide who will fight for the top spot in the Conservative Party.

Governing the process is the party's 1922 Committee.

This is a group of all backbench Conservative Party MPs - whose governing executive body includes Carlisle MP John Stevenson - are not government ministers. They represent the Parliamentary grass roots - the power base - of the party.

Led by Sir Graham Brady and 17 other executive members, the committee is designed to be the 'voice' of rank-and-file Conservative MPs, the most critical job of the 1922 Committee is to oversee the Conservative Party leadership elections.

According to the committee's rules, the first stage in a challenge to the party leader is a vote of no confidence. This is triggered when 15 per cent of Conservative MPs declare they have lost confidence in their current leader.

With 315 Conservative MPs in this Parliament, the number of declarations needed for a vote of no confidence is 48.

It is this number that was reached last night, with the 48th letter of no confidence being received by Graham Brady.

This means there will be a vote held tonight, with all Conservative MPs casting their ballot on Theresa May's leadership.

The Prime Minister needs 50 per cent of her MPs to support her in order to survive tonight's vote.

But if fewer than 158 Conservative Party MPs back Theresa May, she will be ousted as leader of the Conservative Party, bringing her time as Prime Minister to an end.

But one crucial rule in the Conservative Party's handbook is that only one vote of no confidence can be held per year.

Therefore, if Theresa May survives tonight's vote, she will be immune from being challenged by her own MPs for 12 months.

Because of this rule, tonight is the final chance for the Conservative Party's most vocal critics of Theresa May to remove her from office before the completion of Brexit - March 29 2019.

If they are successful, and Theresa May is defeated in tonight's vote, then Conservative MPs will begin to nominate candidates to be their next leader.

If more than two candidates find themselves in the running, then one or more round of elimination elections will be held, with the candidate winning the fewest votes being knocked out.

Once two candidates emerge from this process, the final vote will be put to the general membership of the Conservative Party across the UK.

The winner of this vote will then become the Prime Minister.