Theresa May's turbulent leadership of the Conservative Party will end on June 7, paving the way for a new prime minister to lead the Brexit process.
A tearful Prime Minister said she had "done my best" to get her Withdrawal Agreement through Parliament and take the UK out of the European Union but acknowledged she had failed.
"It is and will always remain a matter of deep regret to me that I have not been able to deliver Brexit," she said in Downing Street.
Watched by husband Philip and her closest aides, an emotional Mrs May said it was in the "best interests of the country for a new prime minister to lead that effort".
Announcing her departure from a job she loved, Mrs May said: "I am today announcing that I will resign as leader of the Conservative and Unionist Party on Friday 7th June so that a successor can be chosen."
Concluding her resignation statement, Mrs May broke down as she said it had been "the honour of my life" to serve "the country that I love".
Earlier, in a sign that the leadership race to replace Mrs May is already under way, Helen Grant quit as Conservative vice chair for communities to "actively and openly" support Dominic Raab.
She quit her Tory party role to avoid any "perception of a conflict" between Mr Raab's campaign and Conservative HQ".
Ms Grant said the former Brexit secretary "has an inspiring vision for a fairer Britain and I think he is undoubtedly the best person to unite the Conservative Party and our country".
Theresa May's legacy will be defined by Brexit chaos:
Theresa May's legacy as Prime Minister will be defined by her fateful decision to call a snap election – and the Brexit chaos that followed.
She arrived in Downing Street on July 13 2016 faced with the task of bringing together party and country after the traumas of the EU referendum.
She will be leaving with her party fractured and the country still divided over Europe.
Her premiership has been dominated by tortuous negotiations in Brussels and vicious infighting within Tory ranks over the terms on which the UK would leave.
Mrs May, 62, marked her arrival with an impassioned promise on the steps of Number 10 to tackle the "burning injustices" which hold back the poor, ethnic minorities, women and the working classes in modern British society.
But her disasterous decision the following year to hold a snap election deprived her of her slim majority in the House of Commons, leaving her dependent on the Democratic Unionist Party (DUP).
From that point on, she was engaged in a day-by-day battle to force her agenda through and maintain the fragile unity of her Government.
She lost more than 30 ministers – most of them quitting over her Brexit plans – saw her keynote policy defeated by a record-breaking 230 votes and suffered the indignity of having her Government found in contempt of Parliament.
It all looked so different when Leave-backing leadership rival Andrea Leadsom dropped out of the contest to succeed David Cameron, clearing the way for former Remainer Mrs May to take office without a vote of Tory members.
Hailed by some commentators as a "new Iron Lady", the vicar's daughter hardened by six years as home secretary immediately showed her ruthless streak, sacking both Michael Gove and chancellor George Osborne, with whom she had clashed in Cabinet.
In her first speech to Conservative conference, she shocked many by setting out "red lines" for withdrawal which put Britain on track for a hard Brexit.
She dismissed her critics as people who saw themselves as "citizens of the world" but were in fact "citizens of nowhere".
Determined to show she was taking the UK into a new global role, she rushed to be the first world leader to meet Donald Trump at the White House after his inauguration in January 2017.
But footage of her holding hands with the US president exposed her to ridicule and raised questions about her closeness to a man whose unpredictability was already causing concern in capitals around the world.
The decision to call an early election in the hope of securing the comfortable majority she needed to implement her Brexit plans was taken on an Easter walking holiday in Snowdonia with husband Philip.
A poorly received manifesto and hastily withdrawn social care policy, coupled with a robotic campaigning style and a surprise outbreak of Corbynmania, saw her squander a 20-point lead in the polls and lose 13 MPs.
When the dust had settled her Tory majority had been wiped out and a visibly distraught Mrs May had to turn to the DUP to prop her up in Parliament, with £1 billion in extra Government funds going to Northern Ireland in return.