"I came into office at a time of great economic and international instability," Truss said in a resignation speech outside Downing Street in London. "Families and businesses were worried about how to pay their bills, Putin's illegal war in Ukraine threatens the security of our whole continent and our country has been held back for too long by low economic growth."
She added: "I was elected by the Conservative Party with a mandate to change this. We delivered on energy bills and on cutting National Insurance. And we set out a vision for a low tax, high growth economy that would take advantage of the freedoms of Brexit."
But she told reporters that, "given the situation," she could not "deliver the mandate" on which she 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," Truss said.
She announced that she would remain as prime minister "until a successor has been chosen."
Truss said in late September that Britain would never accept Russia's attempted annexation of four regions of Ukraine.
Britain's government last month pledged to "match or exceed" its 2022 military support to Ukraine next year.
It said at the time that the UK was the second-largest military donor to Ukraine, after the United States, committing £2.3 billion in 2022, according to the gov.uk website.
Thursday is day 239 of Russia's war on Ukraine.
(gs)
Source: PAP, Reuters, CNN