Liz Truss has resigned as prime minister just 44 days after taking over from Boris Johnson.
She will be the shortest-serving prime minister in British history.
A new leader is set to be elected next Friday.
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.”
Sunak favourite to be new PM as Truss resigns – live politics updates
She said she was elected “with a mandate to change this”, adding: “We delivered on energy bills.”
” I recognise, though, given the situation, I cannot deliver the mandate on which I was elected by the Conservative Party,” she said.
“I have therefore spoken to His Majesty the King to notify him that I am resigning as leader of the Conservative Party.
“This morning, I met the chairman of the 1922 committee, Sir Graham Brady. We’ve agreed that there will be a leadership election to be completed within the next week.”
Ms Truss will remain as PM until her successor has been chosen.
Sir Graham said they expect to conclude a leadership election by Friday 28 October with a new PM in place in time for the 31 October fiscal statement.
He said Tory members are expected to be able to vote but the candidates could be whittled down to just one.
A senior Tory source suggested MPs will have to put their nominations in by Monday.
Ms Truss’ resignation came just a little over 24 hours after she told MPs she was a “fighter, not a quitter”.
Who will be the next PM?
There has been much speculation about who could replace Ms Truss, with new chancellor Jeremy Hunt one of the main names being suggested.
However, Sky News’ deputy political editor Sam Coates said he has been told Mr Hunt will not stand.
Other Tory MPs being suggested are Penny Mordaunt, Rishi Sunak, Kemi Badenoch, and even Boris Johnson.
Former leadership candidate Tom Tugendhat has ruled himself out.