The PM had vowed to carry on, but faced an uphill battle after even his most ardent allies including Michael Gove asked him to step down.
Johnson, who survived a no confidence vote in June, will stay on as PM until a replacement is found.
Here’s everything you need to know about the Conservative leadership race.
When will the Conservative leadership race begin?
The Conservative leadership race is set to take place this summer.
A new leader for the Conservative Party will be elected and will become the Prime Minister, this is expected to be in time for the Conservative Party conference in October 2022.
In the meantime, Johnson will continue to carry out his role as PM, whilst the contest takes place.
There are a number of high-ranking Conservative politicians who are expected to put themselves forward to replace the PM.
Those anticipated to put themselves forward include: former Chancellor Rishi Sunak, former Health Secretary Sajid Javid, Home Secretary Priti Patel, Foreign Secretary Liz Truss and former Foreign Secretary Jeremy Hunt.
What is the process?
Selecting a new leader takes time, with the current process split into three stages.
First, the leadership contest is triggered by either the previous leader stepping down or a vote of no confidence, which Johnson narrowly won back in June 2022.
Second, the 1922 Committee determines candidates based on the Conservative Party Board.
When there is a group of candidates, these are whittled down through ballot voting by Conservative MPs until there are only two candidates left.
However, in the rare case that only one candidate is put forward, they immediately become the winner.
A leader who resigns, is not eligible to run again for the role.
The final and third stage will see Conservative Party members vote for their favourite candidate out of the two.
The one who receives the most votes is declared the winner.
Why has Boris Johnson resigned?
The PM has resigned in the wake of the Chris Pincher groping scandal.
Johnson initially refused to suspend Pincher’s position as Chief Whip and insinuated that he had not been aware of the previous allegations against the MP.
However, after more allegations came to light, it was confirmed that the PM had been aware of the allegations when he appointed him and that Pincher had previously been suspended.
This led to a mass exodus of staff resigning against the PM’s conduct, with over 50 members of the government and his cabinet resigning over Johnson’s handling of the scandal.
The PM has consistently found himself in hot water, from the partygate scandal, to alleged comments about letting “bodies pile high” during the Covid pandemic.
He previously survived a vote of no confidence in June 2022.
Johnson is expected to stay on as PM until Autumn, but there are some MPs who think he should go immediately.
Speaking anonymously to the Guardian one said: “He needs to be gone by tonight, Raab should take over.” Another said: “He needs to hand in the seals of office today and go. So we can have a caretaker PM.”
Whilst another called his move to stay until Autumn “dangerous.”