Michael Gove has dramatically announced he will stand in the Conservative Party leadership contest.
The justice secretary had been expected to support Boris Johnson's bid to succeed David Cameron.
But in a statement released on Thursday morning, Gove took a deliberate swipe at the former London mayor.
"Boris cannot provide the leadership or build the team for the task ahead," he said.
Announcing his bid, Gove 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.
"I have, therefore, decided to put my name forward for the leadership.
Gove had previously said he had no interest in being prime minister or leader of his party.
The justice secretary worked closely with Johnson during the EU referendum leading the Brexit campaign.
The former mayor of London lavished praise on Gove during the campaign, even giving him the nickname "Gover".
Gove's announcement came just minutes before home secretary Theresa May formally launched her leadership bid.
Former defence secretary Liam Fox will also announce he is seeking the leadership today.
Energy minister Andrea Leadsom also announced she would run in the contest this morning.
Work and pensions secretary Stephen Crabb yesterday became the first Conservative MP to announce he was running.
Micheal Gove's Leadership Statement In Full
The British people voted for change last Thursday. They sent us a clear instruction that they want Britain to leave the European Union and end the supremacy of EU law. They told us to restore democratic control of immigration policy and to spend their money on national priorities such as health, education and science instead of giving it to Brussels. They rejected politics as usual and government as usual. They want and need a new approach to running this country.
There are huge challenges ahead for this country but also huge opportunities. We can make this country stronger and fairer. We have a unique chance to heal divisions, give everyone a stake in the future and set an example as the most creative, innovative and progressive country in the world.
If we are to make the most of the opportunities ahead we need a bold break with the past.
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.
I have, therefore, decided to put my name forward for the leadership. 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.