Rishi Sunak has said he has "heard your anger" as he resigned as leader of the Conservative Party after Labour won a landslide in the general election.
Mr Sunak said he would not leave his role immediately but would do so once a Tory leadership race begins.
"I would like to say, first and foremost, I am sorry. I have given this job my all," he said on the steps of 10 Downing Street.
"But you have sent a clear signal that the government of the United Kingdom must change.
"And yours is the only judgement that matters.
"I have heard your anger, your disappointment, and I take responsibility for this loss."
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He added: "I am honoured to have been your prime minister, this is the best country in the world."
The Conservatives have lost more seats than in any other election on record as large swathes of the country turned red.
Sir Keir Starmer claimed victory at 5am on Friday - telling a crowd of supporters that the UK has "the opportunity after 14 years to get its future back".
After winning his seat in Richmond, North Yorkshire, moments before, Mr Sunak conceded defeat and said it was a "sobering" night.
He added: "The Labour Party has won this general election and I have called Sir Keir Starmer to congratulate him on his victory.
"The British people have delivered a sobering verdict tonight, there is much to learn... and I take responsibility for the loss."
After giving his resignation speech in Downing Street, Mr Sunak and his wife Akshata walked up Downing Street into a car that took them to Buckingham Palace where he tended his resignation to the King, as required.
Soon after, the King's spokesman said: "His Majesty was graciously pleased to accept."
Mr Sunak admitted during his resignation speech it "is a difficult day at the end of a number of difficult days".
In a message to the hundreds of Tories who have lost their seats, he said: "I am sorry."
"It pains me to think how many good colleagues who contributed so much to their communities and our country will now no longer sit in the House of Commons. I thank them for their hard work and their service," he added.
Mr Sunak said it is "important" the Conservative Party now rebuilds and also takes up its "crucial role in opposition, professionally and effectively".
The MP wished Sir Keir and his family well, adding: "Whatever our disagreements in this campaign, he is a decent, public-spirited man who I respect.
"He and his family deserve the very best of our understanding as they make the huge transition to their new lives behind this door and as he grapples with this most demanding of jobs in an increasingly unstable world."
He also thanked his Conservative colleagues, his cabinet, the civil service and his teams in Downing Street, the PM's country retreat Chequers and his staff at Conservative headquarters.
Mr Sunak reflected on his leadership, saying inflation is "back to target, mortgage rates are falling and growth has returned".
He said the Conservatives have "enhanced our standing in the world, rebuilding relations with allies, leading global efforts to support Ukraine and becoming the home of a new generation of transformative technologies".
Mr Sunak also said "our United Kingdom is stronger" thanks to the Windsor Framework, devolution "restored" in Northern Ireland and "our union strengthened" - which he said he was proud of.
He ended by talking about Britain being remarkably "unremarkable" due to the fact two generations after his grandparents arrived in the UK he could become prime minister and "could watch my two young daughters light Diwali candles on the steps in Downing Street".
Skynews