Liz Truss has resigned as prime minister just 44 days after taking over from Boris Johnson.
In a statement read outside Downing Street, Ms Truss said: "I came into office at a time of great economic and international instability."
"Families and businesses were worried about how to pay their bills."
She said she was elected "with a mandate to change this", adding: "We delivered on energy bills."
Ms Truss said she recognised she "cannot deliver the mandate" on which she was elected and that there will be a leadership election "to be completed within the next week".
Her time in office has been dominated by market chaos prompted by the mini-budget that was announced by former chancellor Kwasi Kwarteng last month.
Despite sacking Mr Kwarteng last week and reversing almost all the unfunded tax cuts that had been proposed, the prime minister's position had continued to come under pressure.
Some Tory MPs had publicly called for her resignation, but many others had privately suggested her time was up.
While Conservative Party rules prevent a challenge in the first 12 months of a new leader's tenure, it was reported that a significant number of MPs had written to Sir Graham Brady, chair of the 1922 committee, to make clear they had lost confidence in the PM.
On Monday, new Chancellor Jeremy Hunt announced the government would be cutting the energy price guarantee back to six months from the two years previously promised and abandoning the planned 1p reduction to the basic rate of income tax.
The announcement was widely seen as the complete upheaval of Ms Truss's economic programme, central to her leadership bid.
After Mr Hunt's statement, the prime minister's official spokesman refused to deny that Ms Truss was about to resign, instead saying she was "working very closely" with the new chancellor.
On Tuesday, Ms Truss sent House of Commons leader Penny Mordaunt to answer an urgent question in the Commons tabled by Labour on the sacking of Mr Kwarteng.
Ms Mordaunt denied to MPs that Ms Truss was hiding "under a desk".
1:49PM: 'I'm a fighter, not a quitter'
In a calamitous 24-hour period on Wednesday, Suella Braverman lashed out at Ms Truss's "tumultuous" premiership as she resigned as home secretary, after sending secure information using a private email.
Ms Braverman was the second departure from the four great offices of the state within Ms Truss's first six weeks in Number 10.
A few hours later, there was mayhem in the Commons over whether a fracking vote was being considered as a confidence vote in the government and whether the chief and deputy chief whip had quit.
There was speculation that Chief Whip Wendy Morton and her deputy, Craig Whittaker, walked out after a last-minute U-turn on a threat to strip the party whip from Conservative MPs if they backed a Labour challenge over fracking.
Later, Business Secretary Jacob Rees-Mogg had to deny claims made by Labour MP Chris Bryant that some MPs were "physically manhandled into another lobby and being bullied" during the vote.
Mr Rees-Mogg told Sky News he had seen no evidence of anyone being manhandled, but senior Tory MP Sir Charles Walker said what took place was "inexcusable" and "a pitiful reflection on the Conservative Parliamentary Party".
Commons Speaker Sir Lindsay Hoyle confirmed on Thursday morning that he had asked officials to investigate allegations made about incidents in the Commons on Wednesday night.