Sir Keir Starmer has said he will quit as Labour Party leader, paving the way for a contest to decide a new prime minister.
Speaking in Downing Street, Sir Keir said he accepted he was not best placed to lead Labour into the next general election and he had informed the King of his decision to step down.
Sir Keir added he has asked Labour's governing body to set out a timetable to replace him, with nominations opening on 9 July and ending by the summer recess on 16 July.
He said if there was a contest then a new leader would be in place before Parliament returns in September, and he will "do everything" he can to ensure an "orderly" transition of power.
Sir Keir said he would remain as prime minister until the leadership contest is complete.
He added he would also give his successor "my full and unequivocal support, knowing that they will inherit a Britain that is far stronger and fairer than the one I inherited two years ago".
Andy Burnham is regarded by many as the frontrunner to replace Sir Keir after he secured an emphatic win over his Reform UK rival in last week's Makerfield by-election.
Burnham announced on Monday that he would put himself forward as a candidate in the leadership contest, before he boarded a train to London to take his parliamentary seat.
His chances were given an immediate boost by former Health Secretary Wes Streeting, who had been viewed as his main rival, offering his backing to the former Greater Manchester mayor.
Former Armed Forces Minister Al Carns, who has been touted as a possible leadership contender, told ITV's Robert Peston on Monday evening that he was "not ready to make a decision".
Speaking to the BBC as he arrived at Euston station, Burnham praised Sir Keir's "dedication and service".
Asked if he would call a general election in the event that he became prime minister, he replied: "You're jumping several hurdles ahead. My priority is to be sworn in as the MP for Makerfield."
On being formally sworn in as an MP in the House of Commons, Burnham was greeted by loud cheers from Labour benches and a few heckles from the opposition, with one MP shouting: "He's not the messiah."
After taking his seat, he joined around 200 Labour MPs in Westminster Hall to take a group selfie.
Burnham is expected to give a speech next week pledging to grow the economy if he becomes PM, while also sticking to the government's fiscal rules.
His allies say he is yet to decide who would be his chancellor, with one source saying: "No jobs have been given and no deals made."
Sir Keir was elected leader of the Labour Party in April 2020 and became prime minister on 5 July 2024 following Labour's landslide general election victory.
He will leave Downing Street as the shortest-serving Labour prime minister in history.
His period in office will last longer than his Conservative predecessors Rishi Sunak and Liz Truss but behind all six previous Labour prime ministers.
Sir Keir's decision to step down also means the UK will soon have its seventh prime minister since 2016.
Speaking at a lectern in Downing Street, Sir Keir said his party had asked "whether I am best placed to lead us into the next general election".
He said: "I have heard the answer of my parliamentary party to that question, and I accept that answer with good grace."
Sir Keir was accompanied by his wife, Victoria, as he walked out to deliver his resignation speech at 09:30 BST in the blazing sunshine.
Watched by his supporters, colleagues and No 10 staff, Sir Keir's voice cracked with emotion as he spoke of what his focus will be on next.
He said: "When I leave the biggest job in the country, I shall spend more time on the most important job: being the best husband I can to my fantastic wife Vic, who has been a rock by my side through good times and bad; and being the best dad I can to my beautiful children, who are my pride and my joy."
The sound of Beethoven's Ode to Joy could be heard playing in the background as Sir Keir delivered his speech, with the EU anthem being played by a protester.
Sir Keir once described it as the piece of music that best "sums up" his party, telling Classic FM in 2023 that the symphony had a "sense of destiny and is hugely optimistic... it's that sense of moving forward to a better place".
Chancellor Rachel Reeves paid tribute to Sir Keir for helping to "build a stronger, more secure Britain", saying the pair had "achieved a lot together to be proud of, and there is more to do".
Former Labour deputy leader Angela Rayner said "history will remember not just the challenges he faced but the achievements he oversaw", as she pointed to reforms to employment and leasehold legislation.
Burnham thanked Sir Keir for his leadership and said the country now expects "stability, seriousness and a continued focus on the issues that matter most and that is what it will get".
Announcing his widely-expected decision to stand in the leadership contest, he wrote on X: "People want to see progress on economic growth, cost of living, public services, housing and opportunities for the next generation."
Streeting had previously outlined his intention to join any Labour leadership contest", but on Monday said he had "spoken at length with Andy in recent days" and called on colleagues to back Burnham.
Streeting said he was convinced that Burnham "is committed to building an inclusive party that draws on the best of our political traditions" and that he "can win the fight of our lives against the force of nationalism".
Sir Keir had spent the weekend mulling over his future at Chequers, the prime minister's country residence in Buckinghamshire.
Pressure from within Labour had been mounting on Sir Keir to outline a timetable for his departure following Burnham's victory in the Makerfield by-election.
Discontent towards Sir Keir's leadership had also been rising before a poor set of election results in England, Wales and Scotland in May.
This included over his decision to change direction on three major policies in a month after pressure from within his own party.
Sir Keir's decision to appoint Lord Mandelson as UK ambassador to the US also led to questions about his judgement and the wider Downing Street operation.
Lord Mandelson was sacked after new information came to light about the depth of his relationship with the late convicted sex offender Jeffrey Epstein.
Sir Keir opened his resignation speech by defending his record in government, including on employment rights, immigration and child poverty.
He also argued that he had changed Labour after inheriting a party that was "politically, financially and morally bankrupt".
Paying tribute to his outgoing leader, Deputy Prime Minister David Lammy said Sir Keir's record on foreign policy was "second to none".
He told MPs in the House of Commons that the prime minister had been "principled, courageous and on the right side of history".
Asked about Sir Keir's departure, US President Donald Trump said the prime minister was a "very nice man" and "sort of a friend of mine" but criticised his position on the Iran war and drilling for oil and gas in the North Sea.
Conservative Party leader Kemi Badenoch described Sir Keir as a "terrible prime minister" and attacked his policies, including the rise in employer National Insurance contributions and "giving up on real welfare reform".
She wrote on X: "But the problem isn't just Starmer.
"Labour MPs only want higher taxes to hand out more benefits, as the welfare secretary has pointed out. These are Labour's choices and their values, regardless of who is running the party."
Liberal Democrat leader Sir Ed Davey said the British people were "sick of being let down by an endless merry-go-round of prime ministers while nothing really changes".
He said: "This time must be different. It can't just be about changing who's in Number 10, it has to be about changing our broken politics so we can fix our country."
Reform leader Nigel Farage demanded a general election, saying: "If Labour thinks it can shove another professional politician into No 10, it has another thing coming."
Zack Polanski, leader of the Green Party of England and Wales, said people would hope that Burnham can bring about "meaningful change" but the "jury is out".
"The question for Burnham is - are you willing to tax wealth fairly, are you willing to bring our water companies into public ownership, are you willing to bring in PR [proportional representation] so we can have a fair voting system and a better politics that represents everyone."
