The name of Britain's next prime minister should become clearer on Monday when the deadline for entries for the race to succeed Liz Truss comes around, with candidates required to have the support of 100 out of 357 Conservative members of Parliament to be considered.

Penny Mordaunt was the first candidate to confirm that she was running, followed by former chancellor of the exchequer Rishi Sunak, the man who lost to Truss in the summer, before she embarked on the shortest premiership in British history.

"The United Kingdom is a great country, but we face a profound economic crisis," Sunak said in a statement on Sunday morning.

"That's why I am standing to be leader of the Conservative Party and your next prime minister. I want to fix our economy, unite our party and deliver for our country."

The biggest unanswered question of the campaign remains the involvement, or not, of Truss' predecessor Boris Johnson.

Johnson resigned in July, forced out by the Partygate scandal, which saw him being fined by police for breaking pandemic lockdown regulations and widespread criticism of his handling of internal party discipline.

He did not formally relinquish power until early September. And in his resignation speech, the former classics scholar likened himself to the Roman statesman Lucius Quinctius Cincinnatus in how he was quitting politics for a quiet life, although as some observers noted, Cincinnatus was later called back to front-line politics to lead the Roman empire.

"I will be offering this government nothing, but the most fervent support," Johnson also promised in the same speech.

Johnson was on holiday when Truss' premiership came to an end, and returned on Saturday amid a blaze of publicity, but still not saying explicitly whether he would run.

One of the blows that forced Johnson out was when then-chancellor Sunak quit, saying "it has become clear to me that our approaches are fundamentally too different … I have reluctantly come to the conclusion that we cannot continue like this".