Ex-London mayor and prominent Brexit campaigner Boris Johnson has ruled himself out of the race to be the next Conservative leader and prime minister.
In a speech in London - that had billed as his campaign launch - Mr Johnson said he did not believe he could provide the leadership or unity needed.
Justice Secretary Michael Gove had been set to back his fellow Leave campaigner but made a surprise announcement that he would run for leader.
Mr Gove said he was standing because he had come to the conclusion that Mr Johnson could not provide the leadership for the task ahead.
Home Secretary Theresa May, who backed staying in the EU, is among the five candidates and is favourite to win the contest.
The contest was sparked by David Cameron's decision to resign as prime minister after the UK voted to leave the EU and the winner is due to be announced on 9 September.
Other contenders are Work and Pensions Secretary Stephen Crabb, who backed Remain, ex-Defence Secretary Liam Fox, who backed leaving the EU and energy minister and Leave campaigner Andrea Leadsom.
Launching her leadership bid, Theresa May promised to honour last week's decision by voters to take the country out of the European Union.
"Brexit means Brexit," Ms May said in a speech.
"The campaign was fought, the vote was held, turnout was high and the public gave their verdict. There must be no attempts to remain inside the EU, no attempts to rejoin it through the back door and no second referendum."
She said there should be no national election before 2020, the scheduled date for the end of the current parliament, and that there should be no emergency budget measures to offset the expected hit to Britain's public finances from a slowdown in the economy caused by the shock referendum result.
Mr Gove said last week's Brexit result was a vote for "change". The British public "rejected politics as usual and government as usual" and wanted "a new approach to running this country", he said.
Mr Gove said the UK faced "huge challenges" but also "huge opportunities", and added: "If we are to make the most of the opportunities ahead we need a bold break with the past."