Former British Prime Minister Boris Johnson has announced he will not run in the Conservative Party leadership election to choose the successor to Prime Minister Liz Truss.

In a Sunday statement Johnson said, "There is a very good chance that I would be successful in the election with Conservative Party members -- and that I could indeed be back in Downing Street on Friday."

But he went on to say, "I have sadly come to the conclusion that this would simply not be the right thing to do. You can't govern effectively unless you have a united party in Parliament."

Truss announced her resignation on Thursday. Attention had been focused on Johnson's bid to return to office.

Former finance minister Rishi Sunak and leader of the House of Commons Penny Mordaunt have declared their candidacies.

Johnson said he reached out to Sunak and Mordaunt "because I hoped that we could come together in the national interest" but that they could not work out a way to do this. He added, "the best thing is that I do not allow my nomination to go forward and commit my support to whoever succeeds."

To run in the leadership race, candidates need to secure the backing of at least 100 Conservative lawmakers in the House of Commons. Local media outlets report that Sunak has the backing of more than 140 lawmakers and Mordaunt has 20 or so.

If Mordaunt does not reach the threshold of 100 nominations by Monday, Sunak will be named prime minister that day. If Mordaunt does, a party vote will be held and the winner will be announced on Friday.