Liz Truss has dramatically resigned as prime minister after just 45 days in the job.
The PM said her successor will be elected in a Tory leadership contest, to be completed in the next week.
Tory MPs urged Ms Truss to go after her government was engulfed by political turmoil, following the ditching of most of her economic policies.
Ms Truss was elected by the Tory membership in September, but she lost authority after a series of U-turns.
In a brief speech outside Downing Street, Ms Truss said the Conservative Party had elected her on a mandate to cut taxes and boost economic growth.
But given the situation, Ms Truss said: “I recognise that I cannot deliver the mandate on which I was elected by the Conservative Party.”
Ms Truss said she would remain in post until a successor formally takes over as party leader and is appointed prime minister by King Charles III.
Jeremy Hunt – who was appointed chancellor last week – has said he will not stand in the leadership contest to be the next prime minister.
Ms Truss will become the shortest-serving PM in British history when she stands down.
Labour leader Sir Keir Starmer called for an immediate general election following Ms Truss’s resignation speech.
In her speech, Ms Truss said she entered “office at a time of great economic and international instability”, as war rages in Ukraine and living costs skyrocket.
The prime minister said her government delivered on providing support for energy bills and reversing a rise in National Insurance, a tax on workers and companies.
But Ms Truss’s resignation comes after a period of political and economic turbulence, which forced her government to ditch tax cuts that sent financial markets into a tailspin.
The prime minister sacked close political ally Kwasi Kwarteng as chancellor and appointed Jeremy Hunt as his successor as she attempted to calm the markets.
At Prime Minister’s Questions on Wednesday, Ms Truss insisted she was a “fighter, not a quitter” after Labour leader Sir Keir Starmer asked her why she had not resigned.
But more instability followed, when Suella Braverman quit as home secretary and a vote on fracking fell into disarray, with some Tory MPs accused of bullying.