Speaking outside the door of her Number 10 Downing Street office, Truss accepted that she could not deliver the promises she made when she was running for Conservative leader, having lost the faith of her party.
A leadership election will be completed within the next week to replace Truss, who is the shortest serving prime minister in British history. George Canning previously held the record, serving 119 days in 1827 when he died.
"I recognise though, given the situation, I cannot deliver the mandate on which I was elected by the Conservative Party. I have therefore spoken to His Majesty the King to notify him that I am resigning as leader of the Conservative Party," she said.
Earlier, Conservative Party officials had gathered at Downing Street while a growing number of her own lawmakers called on her to quit.
Appointed on September 6, Truss was forced to sack her finance minister and closest political ally, Kwasi Kwarteng, and abandon almost all her economic program after their plans for vast unfunded tax cuts crashed the pound and British bonds. Approval ratings for her and her Conservative Party collapsed.
On Wednesday she lost the second of the government's four most senior ministers, faced laughter as she tried to defend her record to parliament and saw her MPs openly quarrel over policy, deepening the sense of chaos at Westminster.
New finance minister Jeremy Hunt is now racing to find tens of billions of pounds of spending cuts to try to reassure investors and rebuild Britain's fiscal reputation as the economy heads into recession and with inflation at a 40-year high.
