Justice Secretary Michael Gove is to run to be the next Conservative Party leader and UK prime minister.
Mr Gove, a prominent figure in the Brexit campaign, had been expected to support Boris Johnson's candidacy.
He said he was standing because he had come "to the conclusion that Boris cannot provide the leadership or build the team for the task ahead".
Home Secretary Theresa May, who backed staying in the EU, has also entered the contest. Nominations close at noon.
The winner of the contest is set to be announced on 9 September.