Michael Gove has said he chose to run for the Conservative Party leadership after deciding “reluctantly but firmly” that Boris Johnson was not capable of uniting the party or the country.
“It had to fall to someone else… I felt it had to fall to me,” he said.
The justice secretary was set to back his fellow Leave campaigner. Mr Johnson pulled out after Mr Gove’s switch.
Home Secretary Theresa May, who backed staying in the EU, is favourite to win the contest and become prime minister.
The other contenders are Work and Pensions Secretary Stephen Crabb, who backed Remain, former defence secretary Liam Fox, who backed leaving the EU, and energy minister and Leave campaigner Andrea Leadsom.
Nominations for the Conservative Party leadership closed at midday on Thursday.
The winner of the contest – sparked by David Cameron’s decision to resign after the UK voted to leave the EU – is set to be announced on 9 September.
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Mr Gove’s decision to run for the leadership and Mr Johnson’s subsequent decision to bow out of the contest sent shockwaves through the political world in a day of high drama.
In an interview with the BBC political editor Laura Kuenssberg, Mr Gove, explaining his reasons for standing, said following last week’s Brexit result he felt the country needed a leader “who believed heart and soul in leaving the European Union”.
“I also believed we needed someone who would be able to build a team, lead and unite. I hoped that person would be Boris Johnson,” he said.
But he added: “I came in the last few days, reluctantly and firmly, to the conclusion that while Boris has great attributes he was not capable of uniting that team and leading the party and the country in the way that I would have hoped.”