The former prime minister said he had “reached out” to leadership rivals Rishi Sunak and Penny Mordaunt to see if they could work together in the national interest, but it had not proved possible.
In a statement, he insisted he had secured the 100 nominations needed to get onto the ballot paper – and that if he stood there was a “very good chance” he would be back in Downing Street by the end of the week.
He went on: “But in the course of the last days 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.
“And though I have reached out to both Rishi and Penny – because I hoped that we could come together in the national interest – we have sadly not been able to work out a way of doing this.
“Therefore I am afraid the best thing is that I do not allow my nomination to go forward and commit my support to whoever succeeds.”