Boris Johnson no confidence vote: the facts and figures behind tonight's crucial ballot

He needs to win the support of half his Parliamentary party to remain as leader.

Here are some of the facts and figures that have led up to this moment.

In order for there to be a confidence vote in the leadership of the Conservative Party, 15 per cent of the Parliamentary party must write to the chair of the 1922 Committee of backbench MPs to make clear their desire for there to be a vote.

Given the number of Tory MPs, it meant that Sir Graham Brady had to receive 54 letters of no confidence.

The only person who knows exactly how many letters have been received is Sir Graham, and he suggested earlier on Monday that a number of colleagues had dated letters in such a way as to not disturb the Jubilee celebrations over the weekend.

At least 50 per cent of his 359 MPs must vote ‘no confidence’ in the Prime Minister for him to lose the leadership.

According to a tally from the Institute for Government thinktank, there are around 160 - 170 MPs on the Ministerial payroll, which would get Mr Johnson very close to victory if all of them vote in his favour.

Conservative Party rules state that if the leader wins a confidence vote another cannot be held against them for another 12 months. However, Sir Graham said earlier that “technically it’s possible for rules to be changed but the rule at present is there would be a period of grace.”