Britain's next prime minister will be either ex-finance minister Rishi Sunak or current Foreign Secretary Liz Truss.
But the decision on who will be Boris Johnson's successor is no longer in the hands of Tory MPs, or even the general public.
Instead, thousands of Conservative Party members will spend the summer casting their ballots to decide who gets the top job.
So, who are the Conservative Party members? And who are they likely to vote for?
Who are the Conservative Party members?
There are between 180,000 to 200,000 Conservative Party members across Britain, although the party has chosen not to tell the Electoral Commission its exact membership figures.
Members of the public can join the Conservative Party by signing up through their website and paying an annual fee of £25 ($44), with a discounted rate available for serving and former members of the Armed Forces.
Members can then play an active role in the Conservative Party by being able to attend conferences and receive voting rights in Party elections – which is what's happening now.
But members can only vote after three months of membership, meaning anyone signing up today will not have a say in who becomes Britain's next prime minister.
Now, Rishi Sunak and Liz Truss will have the chance to sway Conservative Party members at 12 hustings events around the country between July 28 and August 31, including in Leeds, Cardiff, Manchester, Birmingham and London.
These events will also be streamed online.
When do Conservative Party members have to vote?
Party members who joined on or before June 3 are eligible to vote in the contest and will receive their ballot papers between August 1-5.
Members can vote by post or online. The last vote received is the one that counts if there is any duplication. Ballots must be received by 4pm on September 2.
On September 5, the party will announce who has been elected by members as the new leaders, with either Rishi Sunak or Liz Truss becoming the next prime minister of Britain.
Boris Johnson is then expected to leave office on September 6 by travelling to Buckingham Palace to tender his resignation to Queen Elizabeth II.
So who will win?
The race is still looking wide open.
Mr Sunak, who helped steer the economy through the pandemic, might not find a forgiving crowd among party members with many blaming him for triggering Mr Johnson's downfall with his resignation earlier this month.
He has also faced criticism on everything from his record in government to his wife's wealth.
Polls show Ms Truss would beat Mr Sunak in the party members' contest, opening up the chance that Conservatives elect a leader who was not the most popular choice for MPs.
But Ms Truss might struggle at the hustings against Mr Sunak, who is more relaxed in public appearances.
On Sunday, she admitted she might not be "the slickest presenter" but "when I say I'll do something, I do it."
Chris Hopkins, the political research director at the polling company Savanta ComRes, says there is no clear winner.
"This has been very different to recent contests where you have had one clear favourite run away with it."