His rival Theresa May joined Twitter yesterday as she began her campaign
The British Justice Secretary Michael Gove has joined the race to be the next Tory leader, saying he does not believe Boris Johnson can "provide the leadership" for the task ahead.
He will make the first speech of his campaign to become the leader of the Conservative Party and the next British Prime Minister later.
Mr Gove, who had repeatedly said he had no leadership ambitions, dropped the bombshell just hours before the midday nominations deadline on Thursday.
His surprise declaration was seen as a devastating blow for Mr Johnson, who was expected to stand with the backing of Mr Gove and who shortly afterwards equally stunned Tory MPs and journalists by declaring he would not be standing in the contest.
The pair campaigned side by side during the EU referendum campaign to secure the shock Brexit victory.
But Mr Gove said events since last week had "weighed heavily with me" and led him to change his mind.
"I respect and admire all the candidates running for the leadership," he said in a statement.
"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."
Just weeks ago, he had insisted "count me out" and told voters: "I'll tell you this, whatever posters you put up on your wall, don't put one up of me."
"I'm not the sort of person you want to have when you wake up, on your bedroom wall."
However, on the eve of the nominations deadline, in a leaked email his wife, journalist Sarah Vine, warned him about the risks of backing the former London mayor without "specific assurances" about his role in any Johnson-led Cabinet.
Minutes after Mr Gove declared his bid, another Brexiteer, Andrea Leadsom, announced she was throwing her hat into the ring also, tweeting: "Let's make the most of the Brexit opportunities!"
And she was swiftly followed by UK Home Secretary Theresa May, who pledged to unite both the Conservative Party and the country.
"My pitch is very simple: I'm Theresa May and I believe I'm the best person to be Prime Minister," she said.
"We need a bold new positive vision for the future of our country - a country that works not for a privileged few but for every one of us."
Mrs May backed Remain - but did so in a less provocative way than many of her senior Tory colleagues and was careful not to alienate either side.
In a move to garner the support of the eurosceptics in her party, she asserted: "Brexit means Brexit...there must be no attempts to remain inside the EU, no attempts to rejoin it through the back door and no second referendum."
"The country voted to leave the European Union and it is the duty of the Government and Parliament to make sure we do just that."
In a clear swipe at Mr Johnson, Mrs May said politics was not a "game" and added: "It's a serious business that has real consequences for people's lives."
She said there should be no general election until 2020 and no "Brexit budget", adding Article 50 - which sets into motion the two-year window for leaving the EU - should not be triggered before the end of this year.
"You can judge me by my record," she said.
"We have immediate work to do. Together, we, the Conservative Party, can build a better Britain."
Asked if there would be a job for Mr Johnson in her Cabinet if she were to win, she replied: "I am being clear I am not going around offering jobs to individuals of any sort."
She also brushed off comparisons with Margaret Thatcher and Angela Merkel as a woman aspiring to lead her country, saying: "Whether it's a woman or a man, what counts is the quality of the individual."
Mrs May later joined Twitter and unveiled the slogan "a country that works for everyone" as she posted her first message.
On Wednesday, former defence British secretary Liam Fox and the Work and Pensions Secretary Stephen Crabb - whose running mate is Business Secretary Sajid Javid - put themselves forward.
Health Secretary Jeremy Hunt and Education Secretary Nicky Morgan, who had both previously indicated they were considering a bid, ruled themselves out of the race and said they would be backing Mrs May and Mr Gove, respectively.