The race is on to become leader of the Conservative Party and Britain's new prime minister.

Prime Minister Theresa May formally stepped down as the leader of the party yesterday, after thrice failing to get her controversial exit deal through Parliament and deliver Brexit.

She will remain in Downing Street as Prime Minister and acting Conservative Party leader during the leadership contest until a successor is chosen. The party said a new prime minister would be in place by the week beginning July 22.

While nominations will take place next Monday, 11 Members of Parliament have already indicated their intention to replace Mrs May, with front-running backbencher and former foreign secretary Boris Johnson officially throwing his hat into the ring five days ago.

Earlier this week, the ruling party overhauled its leadership election rules in a bid to speed up the selection process and prevent too many candidates from standing.

Each candidate must now be backed by eight MPs, up from just two previously.

Nominations must be submitted by 5pm London time on Monday.

The number of contenders will be whittled down to two with a series of votes. Hustings for all nominees will be held in Parliament on Tuesday and Wednesday.

Those vying for the premiership will need the support of at least 17 MPs - equating to 5 per cent of the parliamentary party - in the first round of ballots on Thursday.

To proceed further, candidates will then need 33 MPs - 10 per cent of Tory MPs - to vote for them in the second round on June 18.

Should the thresholds be met by all candidates, the nominee with the fewest votes will be eliminated.

Subsequent rounds of successive balloting will take place on June 19 and June 20, if necessary, until only two contenders remain.

The final two candidates will participate in a series of hustings around Britain from June 22.

About 124,000 Conservative Party members, made up of members of the public who pay to join, will then cast their votes in a postal ballot. As members, they are allowed to choose the candidates who will stand in local and parliamentary elections, as well as vote in leadership elections. They can also stand for election as Conservative councillors and prospective parliamentary candidates, subject to assessments and approvals.

The winner of the contest will become the next leader of the party and the country's prime minister.

A leadership contest can be triggered in two ways: If 15 per cent of Conservative MPs write to the chairman of the 1922 Committee expressing they no longer have confidence in the party leader, or if the current leader resigns.

Last December, Mrs May survived a vote of no confidence.

This meant she could not be challenged for another 12 months.

In an emotional address outside No. 10 Downing Street last month, Mrs May announced her intention to stand down as party leader to pave the way for a new head to see the country through Brexit.