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.
- Follow the latest developments on our live page
- Tory leadership: Runners and riders
- In full: Michael Gove’s statement
Mr Johnson, the former mayor of London, is widely expected to enter the race with a speech later, and to place Brexit at the heart of his “optimistic vision” for the country.
He is expected to say the greater self-determination that leaving the EU will bring is an opportunity to “believe in ourselves and the values of our country”.
Energy minister and Brexit campaigner Andrea Leadsom has also thrown her hat in to the ring, joining former Defence Secretary Liam Fox and Work and Pensions Secretary Stephen Crabb among the runners and riders.
‘Unity and change’
Mr Gove’s announcement that he will challenge the leadership was unexpected, as the justice secretary had been expected to throw his weight behind fellow leading Leave campaigner Mr Johnson for Conservative leader.
Explaining his decision, he said: “I have repeatedly said that I do not want to be prime minister. That has always been my view. But events since last Thursday have weighed heavily with me.
“I respect and admire all the candidates running for the leadership. In particular, I wanted to help build a team behind Boris Johnson so that a politician who argued for leaving the European Union could lead us to a better future.
“But I have come, reluctantly, to the conclusion that Boris cannot provide the leadership or build the team for the task ahead.”
Setting out his pitch for the leadership, the cabinet minister – who was formerly the education secretary – said: “I want there to be an open and positive debate about the path the country will now take.
“Whatever the verdict of that debate I will respect it. In the next few days I will lay out my plan for the United Kingdom which I hope can provide unity and change.”
Mr Gove said the British public, in voting to leave the EU by 52% to 48%, last week, “rejected politics as usual and government as usual” and wanted “a new approach to running this country”.
The UK faced “huge challenges” but also “huge opportunities”, he said, and added: “If we are to make the most of the opportunities ahead we need a bold break with the past.”
‘Bold, positive vision’
Launching her leadership bid in central London, Mrs May – one of the longest-serving home secretaries in history – said the UK needed “strong proven leadership to steer us through this period of economic and political uncertainty and to negotiate the best possible terms as we leave the European Union”.
Although she backed Remain she maintained a low profile during the campaign and, in her speech, insisted she would not back away from last week’s vote. “Brexit is Brexit.”
But she said Article 50 – the formal mechanism for leaving the EU – should not be triggered until the UK had agreed its negotiating strategy – probably before the end of year.
She set out plans for a new government department to oversee the UK’s withdrawal from the EU – and said it would be headed by a pro-Brexit cabinet-level minister.
Mrs May also said that, as prime minister she would drop her call for Britain to withdraw from the European Convention on Human Rights, saying she did not expect there to be a parliamentary majority for it.
Mrs May – the daughter of a vicar – also pitched herself as the candidate that could “unite our party and our country”.
With Labour “tearing itself to pieces” and “divisive nationalists in Scotland and Wales” it was “the patriotic duty of our party to unite and govern in the best interests of the whole country”, she said.
“And third,” she added, “we need a bold, new positive vision for the future of our country. A vision of out country that works not for a privileged few, but for every one of us.”
Mrs May acknowledged she was “not a showy politician”, adding: “I just get on with the job in front of me,” – suggesting that was what the country wanted.
In a swipe at potential leadership rival Mr Johnson, joking: “Boris negotiated in Europe. I seem to remember last time he did a deal with the Germans, he came back with three nearly-new water cannon.”
The then London mayor purchased the riot control water cannon following the riots in the capital city in 2011 – but Mrs May eventually withheld permission for their use.