The decision by Johnson — who spearheaded the “Leave” campaign — upended the Conservative Party leadership contest and added to deep uncertainty over how the country will split from the 28-nation alliance.

Johnson had been widely tipped as a favourite to succeed Cameron, who resigned in the wake of the June 23 referendum and left it to his successor to open formal talks with the EU on Britain’s departure.

The most likely choice is now Interior Minister Theresa May, who called for “unity” in the deeply divided Conservative Party and said Brexit negotiations should not be started this year.

Johnson made his announcement after Brexit ally Michael Gove announced his own bid for the top job, slamming the leadership of the mop-haired politician.

“Having consulted colleagues and in view of the circumstances in parliament, I’ve concluded that person (new prime minister) cannot be me,” Johnson said in a speech in London.

– Mood of betrayal -Justice minister Gove, who campaigned alongside Johnson for the historic vote, said the former London mayor “cannot provide the leadership or build the team for the task ahead”.

Reflecting the mood of betrayal, Johnson quoted from Shakespeare’s Julius Caesar, paraphrasing the final speech given by plotter Brutus shortly before knifing the emperor.

May, a low-key supporter of remaining in the European Union, said she could now provide the “strong, proven leadership to steer us through this period of economic and political uncertainty”.

She stressed that any attempts to “wriggle out” of promises to cut immigration “especially from leadership candidates who campaigned to leave the EU by focusing on the issue” would be unacceptable, interpreted as a swipe at Johnson.

Cameron announced his resignation within hours of last Thursday’s 52-48 percent vote to leave the EU, a decision that has exposed deep rifts in British society and plunged the country into extraordinary political turmoil.

A new leader is set to be voted in by Conservative members by September 9 but European leaders have pressed for a quicker timetable, warning against a prolonged period of uncertainty over Britain’s EU future.

– ‘Global growth concerns’ -May said if she became leader, Britain would not trigger Article 50 of the EU’s Lisbon Treaty, which sets off a two-year timetable for its departure, before the end of the year, saying the country needed to decide what it wanted from a deal.

Work and pensions minister Stephen Crabb and energy minister Angela Leadsom also announced their intention to run.

The Brexit vote has sent the pound plunging and led US President Barack Obama to warn of “longer-term concerns about global growth”.

However, London’s FTSE 100 has staged a strong recovery over the last 48 hours, closing higher on Wednesday than its level before the June 23 referendum.

The outcome, after an often acrimonious campaign, triggered anger among those who wanted to remain in the EU, many of whom believe “Leave” voters were misled and are calling for a second referendum.

However, 58 percent of Britons believe the result should stand against 31 percent who think the vote should be re-run, according to a YouGov poll.

May insisted that “Brexit means Brexit”, and that there would be no backtracking on the vote.

European leaders, keen not to encourage burgeoning anti-EU movements across the continent, have warned that Britain will not be able to pick and choose the cosiest terms of divorce.

German Chancellor Angela Merkel said Wednesday that Britain would have to accept free movement of people within the bloc if it wanted access to the free market, but French Finance Minister Michel Sapin told the BBC that “everything will be on the table” when negotiations begin.

– Labour turmoil -Britain’s main opposition Labour party has also been thrown into turmoil by the vote, as lawmakers moved in its aftermath to oust left-wing leader Jeremy Corbyn in a slow-motion revolt.

Senior lawmaker Angela Eagle is expected to announce Thursday a leadership challenge to Corbyn, who has refused to resign despite losing a confidence vote by MPs.

Though Labour lawmakers passed a vote of no confidence against him by 172 to 40, Corbyn insists he still has the support of the party membership and will fight any leadership challenge he sees as a plot by the more right-wing elements of his party.

“The mandate was given by hundreds of thousands of ordinary people… I’m very proud to be carrying on with that work,” Corbyn told the cheering young activists who form his support base.

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