Political Editor Robert Peston reports on the resignation - and the chaotic days that came before it
Liz Truss has resigned as Prime Minister after just 45 days in office.
In a statement at Number 10, Ms Truss announced she was stepping down as leader of the Conservative Party after a tumultuous premiership, the shortest in British history.
She said a leadership election to replace her would be "delivered within the next week" after meeting with King Charles to offer her resignation. A fresh leadership contest means the UK faces its fifth prime minister in six years.
Just a little over 24 hours earlier, Ms Truss had told MPs she was a “fighter, not a quitter”.
Ms Truss' resignation comes after a chaotic day at Westminster on Wednesday that left Liz Truss battling open revolt from a rising number of Tory MPs.
Speaking at the lectern in Downing Street on Thursday, Ms Truss said she was elected by the Conservative Party with a mandate to change the situation facing the UK.
"We delivered on energy bills and on cutting national insurance.
Robert Peston sums up an extraordinary 45 days in 45 seconds
"We set out a vision for a low-tax, high-growth economy that would take advantage of the freedoms of Brexit."
But she said "the situation" had prevented her from delivering on that mandate.
Sir Graham Brady, the head of the Conservative backbench 1922 committee, said on Thursday that candidates to replace the outgoing PM will need at least 100 nominations from fellow MPs to move forward in the contestpm on Monday.
During the last leadership contest, candidates had to secure a minimum of 20 nominations.
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The rule change means that a maximum of three candidates will be able to stand - and it's been suggested that there will be one hustings event. There will also be an 'indicative vote' of the final two candidates, so party members know who is favoured by MPs.
Conservative party members will then vote online.
Potential candidates to replace Liz Truss had been mooted over the last days of her premiership, including Rishi Sunak and Penny Mordaunt. There have also been calls among some Tory MPs to bring back Boris Johnson, who remains popular among Conservative members.
Who might replace Liz Truss?
Former Cabinet minister Simon Hart said Rishi Sunak would be the “right prime minister”.
He said: “After the last few weeks the very least we can do for the country is get the right prime minister this time.
“This means choosing someone serious, tested, competent and kind. For me that’s Rishi Sunak.”
Ben Chapman spoke to residents of Northampton about their views on the resignation
MP for Peterborough Paul Bristow told ITV News he would back Mr Johnson, who resigned in July, to return as party leader.
"We need a proven election winner and the only person I can see is Boris Johnson," he said.
Former culture secretary Nadine Dorries also said she is backing Boris Johnson to succeed Liz Truss as Tory leader.
Ms Dorries said she had spoken to the former prime minister – who is currently believed to be on holiday in the Caribbean – following Ms Truss’s announcement but declined to say whether he had decided to run.
Both the Times and the Telegraph have since reported that Mr Johnson is considering running.
But he would face a difficult reception from many Tory MPs, after being forced out of office following a number of scandals. One such critic, senior MP Sir Roger Gale, tweeted: "We need to remember that Mr Johnson is still under investigation by the Privileges Committee for potentially misleading the House".
Some opposition figures are calling for him to be barred from running.
“The fact that Conservative MPs are even considering putting Boris Johnson back in Number 10 shows how out of touch they really are,” said Lib Dem deputy leader Daisy Cooper.
“They think there’s one rule for them and another for everyone else.
The pound rallied after Ms Truss's resignation, with sterling 0.4% higher at 1.126 US dollars after her statement.
Neil Wilson from global broker Markets.com said the pound’s “kneejerk verdict was damning”, signalling that her move has been welcomed across the City.
The pound suffered a tumultuous past two months sparked by the market turmoil seen in the wake of former chancellor Kwasi Kwarteng’s mini-budget. At one stage, sterling slumped to its lowest ever level against the US dollar.
Former prime minister Theresa May said Tory MPs must be prepared to compromise to ensure there is a “sensible, competent government”.
Meanwhile, Keir Starmer, leader of the Labour Party, said the Tories "no longer has a mandate to govern" and called for an immediate general election.
“After 12 years of Tory failure, the British people deserve so much better than this revolving door of chaos. In the last few years, the Tories have set record-high taxation, trashed our institutions and created a cost-of-living crisis," he said in a statement.
"Now, they have crashed the economy so badly that people are facing £500 a month extra on their mortgages. The damage they have done will take years to fix."
Scotland First Minister Nicola Sturgeon also called for a general election, insisting a public vote would be a "democratic imperative".
"There are no words to describe this utter shambles adequately," the First Minister said on Twitter.
"It's beyond hyperbole - & parody. Reality tho is that ordinary people are paying the price."The interests of the Tory party should concern no-one right now."
Liberal Democrat leader Sir Ed Davey and First Minister of Wales Mark Drakeford also echoed this sentiment.Mark Drakeford said Ms Truss' time in office had been a "complete and utter failure of government with everyone in this country now having to pay the price."
Internationally, US president Joe Biden said in a statement: “The United States and the United Kingdom are strong Allies and enduring friends — and that fact will never change.
“I thank Prime Minister Liz Truss for her partnership on a range of issues including holding Russia accountable for its war against Ukraine."
While French President Emmanuel Macron said: "I won't comment on this issue which relates to British politics, but what I want to say is that we always had very constructive meetings and exchanges over the phone, no later than a few days ago in Prague.
"I also want to say that France, as a friend of the British people, wishes for stability and in this context of war and tensions over the energy crisis, it is important that Great Britain sets out again on the path of political stability and that's all I wish for."
But the Irish foreign minister Simon Coveney has spoken of his frustration at “being back to instability again” in terms of relations with the UK.
“The meetings between Liz Truss and the Taoiseach were also much better in terms of tone than we had seen for some time,” Mr Coveney told Irish broadcaster RTE.
“The frustration for us is we are back to instability again, I had the privilege of being foreign minister for five years, in that time I have dealt with six secretaries of state for Northern Ireland, five foreign secretaries and now it’s going to be four prime ministers."
Ms Truss officially became the UK's third ever female premier after meeting with the Queen at Balmoral Castle on September 6, the day after it was announced she had beaten rival Rishi Sunak in the Conservative Party member vote. Two days later, she unveiled her first major policy, the emergency support package, in a debate interrupted by news of the Queen's ailing health.
The Queen's death was announced hours later, and parliament was suspended during the official mourning period. The week MPs returned, then chancellor Kwasi Kwarteng set out his mini budget to the Commons that unleashed market turmoil that ultimately lead to the end of both his and Liz Truss's time in office.
Ms Truss has currently clocked up 44 full days in the role – a long way behind the next shortest premiership, that of Tory statesman George Canning, who spent 118 full days as PM in 1827 before dying in office from ill health.
She was to have overtaken this number of days on January 3, 2023.
But instead she will fall short by more than two months, with the next prime minister due to be elected within the next week.