Britain's Rishi Sunak said he would meet King Charles to formally resign as prime minister on Friday and would also stand down as leader of the defeated Conservative Party after Keir Starmer's Labour Party won a landslide election victory.

"I have given this job my all, but you have sent a clear signal that the government of the United Kingdom must change, and yours is the only judgment that matters," Sunak said in a speech to voters outside the prime minister's office in Downing Street.

"I have heard your anger, your disappointment and I take responsibility for this loss. To all the Conservative candidates and campaigners who worked tirelessly, but without success, I'm sorry that we could not deliver what your efforts deserved."