He addressed the nation from outside 10 Downing Street on Thursday (7 July) afternoon.
A group of MPs gathered in Downing Street to show support for Mr Johnson including Nadine Dorries, Jacob Rees-Mogg and Johnny Mercer.
His wife Carrie and his youngest child were also part of the gathering.
His decision to step down as leader of the Conservatives came after his support within the party collapsed over the past 48 hours.
It sparked a flurry of resignations as well as calls for him to resign.
A No 10 source earlier said Mr Johnson spoke to Sir Graham Brady, chairman of the Conservative 1922 Committee, this morning to inform him of his decision to resign.
“The Prime Minister has spoken to Graham Brady and agreed to stand down in time for a new leader to be in place by the conference in October,” they said.
Why has Boris Johnson resigned as Prime Minister?
Boris Johnson quit as Tory leader after admitting he failed to persuade Cabinet colleagues that he could fight on.
The Prime Minister said it was “eccentric” to change governments at this stage but “I regret not to have been successful in those arguments”.
More than 50 ministers have resigned from his government over the past 48 hours.
The Prime Minister had sought to defy his critics and carry on in office, despite warnings from Cabinet colleagues that this was not sustainable.
But the resignations today (7 July) continued and Chancellor Nadhim Zahawi – who was only appointed to the role on Tuesday – went public with his call for the prime minister to quit.
The loss of support in his own party comes after Mr Johnson’s leadership was dogged by a series of scandals.
Who will replace him as prime minister?
Mr Johnson intends to remain in No 10 until his successor is elected.
He has already appointed new Cabinet ministers to replace MPs who quit as part of the mass ministerial exodus in protest at his leadership.
Mr Johnson said: “It is clearly now the will of the parliamentary Conservative Party that there should be a new leader of that party and therefore a new prime minister.
“And I’ve agreed with Sir Graham Brady, the chairman of our backbench MPs, that the process of choosing that new leader should begin now and the timetable will be announced next week.
“And I’ve today appointed a Cabinet to serve, as I will, until a new leader is in place.”
However he faces resistance to his plan to stay on as interim prime minister from within his own party and the Opposition.
Labour will call a no-confidence vote in the Prime Minister if the Conservative Party does not get rid of him immediately, Angela Rayner has said.
Lib Dems have also called for Conservative MPs to “get rid of him today”.
Tom Tugendhat has launched his bid to replace Mr Johnson.
Ben Wallace, Penny Mordaunt, Rishi Sunak, Liz Truss and Nadhim Zahawi are among the early favourites to replace Mr Johnson as leader of the Conservative party.
Attorney General Suella Braverman announced her intention to run for leadership on Wednesday (6 July) evening.
It has also been suggested Grant Shapps could also throw his hat into the ring.
Resignation speech explained - key points
Speaking from a lectern outside 10 Downing Street, Mr Johnson acknowledged: “In politics, no one is remotely indispensable.”
But in a sign of the resentment he feels about being forced from office, less than three years after a landslide election win, Mr Johnson said: “In the last few days, I tried to persuade my colleagues that it would be eccentric to change governments when we’re delivering so much and when we have such a vast mandate and when we’re actually only a handful of points behind in the polls, even in mid-term after quite a few months of pretty relentless sledging and when the economic scene is so difficult domestically and internationally.
“I regret not to have been successful in those arguments and of course it’s painful not to be able to see through so many ideas and projects myself.
“But as we’ve seen, at Westminster the herd instinct is powerful, when the herd moves, it moves.”
He said his successor’s priorities should include “cutting taxes because that is the way to generate the growth and the income we need to pay the great public services”.
“To that new leader, I say, whoever he or she may be, I say: ‘I will give you as much support as I can’.”
What scandals has Boris Johnson’s government faced recently?
This week’s tsunami of resignations came after the Prime Minister was heavily criticised for his handling of the Chris Pincher scandal.
The deputy chief whip quit on Friday 1 July following allegations that he groped two men in a private members’ club.
Mr Johnson originally resisted removing the whip from Mr Pincher, before changing his mind.
He also denied being aware of previous allegations against Mr Pincher and then later claimed to have simply “forgot” about them.
His handling of this latest scandal served as the catalyst for the mass resignations.
He had previously faced criticism for his handling of the Owen Paterson affair, who was accused of breaking lobbying rules for MPs.
Mr Paterson eventually resigned sparking a by-election in North Shropshire, which was won by the Liberal Democrats.
Mr Johnson was then rocked by the Partygate allegations in late 2021 and early 2022, which revealed a number of Covid rule breaching parties at Downing Street during lockdowns.
The Prime Minister himself was fined for attending a gathering on his birthday, Mr Sunak also received a fine.
He was accused of misleading Parliament over statements saying he was not aware of any rule breaking in Parliament.
Mr Johnson survived a no confidence vote in early June by 211 to 148, a far smaller margin than he had hoped for.
The Tories then lost two more by-elections in Tiverton and Honiton and Wakefield to the Lib-Dems and Labour respectively.
How long has Boris Johnson been Prime Minister?
Mr Johnson became Prime Minister on 23 July 2019, succeeding Theresa May - meaning he has been in the role for just shy of three years.
As of 7 July, he has been in office for 1,079 days but Mrs May clocked up 1,106 days in the job between 2016 and 2019.
If he makes it to the end of August, he will have passed two more: Theresa May on August 4 and Jim Callaghan, Labour prime minister from 1976 to 1979, on August 22.