The Conservative Party, which holds a big majority in parliament and need not call a nationwide election for another two years, will now elect a new leader by Oct. 28 - Britain's fifth prime minister in six years.
That contest is likely to pit ex-finance minister Rishi Sunak against Penny Mordaunt, but could also see the return of Boris Johnson, who was ousted as prime minister in July when his ministers resigned en masse to force him out of office.
The sight of yet another unpopular prime minister making a resignation speech in Downing Street - and the start of a new leadership race - underscores just how volatile British politics has become since the 2016 vote to leave the European Union.
Speaking outside the door of her Number 10 office, Truss accepted that she had lost the faith of her party and would step down next week. The pound rallied as she spoke.
"I have therefore spoken to His Majesty the King to notify him that I am resigning as leader of the Conservative Party," said Truss, who was supported only by her husband with her aides and loyal ministers noticeably absent.
Allied leaders said they would continue to work with her successor and emphasised the importance of stability.
