Former UK finance minister Rishi Sunak appears to be well-placed to become the UK's next Prime Minister, as his former boss Boris Johnson steps away from the fresh contest to lead. 

He is favorite to lead the Conservative Party, after Liz Truss resigned, and therefore also take the keys to the famous black door at Number 10. Another former cabinet colleague Penny Mordaunt says she will continue battling for the support to take the top job, but after Johnson's surprise admission that he would not run, Sunak is solidly in the lead. 

Sunak quit Johnson's government in July, helping trigger a rebellion that bought him down.

"There will be integrity, professionalism and accountability at every level of the government I lead and I will work day in and day out to get the job done," he said in a statement.

"I am asking you for the opportunity to help fix our problems."

Sunak has the support from 100 Conservative MPs needed to make it to a vote by the party's members, but if no other candidate reaches that magic number, then the former minister will be 'crowned' leader without an election. 

But a spokesperson for Mordaunt, who also stood in the campaign which Truss won, said she believes the race is not yet run. 

"Penny is speaking to colleagues from across the party. She's getting the numbers and she's in it to win it," the spokesperson said.