LONDON — Britain’s victorious anti-EU campaigner Boris Johnson saw his chances of leading his country evaporate Thursday after the defection of a key ally.

The former London mayor dropped his campaign to become Conservative Party leader and prime minister after Justice Secretary Michael Gove abruptly withdrew his support for Johnson and announced he would run himself.

Johnson, a prominent campaigner for Britain’s withdrawal from the 28-nation European Union, told a news conference where he was expected to announce his candidacy that the next Conservative leader would need to unite the party and ensure Britain’s standing in the world.

“Having consulted colleagues and in view of the circumstances in Parliament, I have concluded that person cannot be me,” he said to the astonishment of gathered journalists and supporters.

Johnson paraphrased Shakespeare’s “Julius Caesar,” saying it was “a time not to fight against the tide of history but to take that tide at the flood and sail on to fortune.” It appeared to be a dig at Gove – the reference is to a line spoken by Brutus, the Roman leader’s ally turned assassin.

Others drew a more contemporary parallel.

“It makes ‘House of Cards’ look like ‘Teletubbies,”‘ Conservative lawmaker Nigel Evans told the BBC.

Johnson’s departure makes Gove and Home Secretary Theresa May the favorites among five contenders to lead the Conservatives.

It is an unexpected twist in a career that has seen the 52-year-old Johnson serve as journalist, lawmaker and mayor, building a public profile on Latin quips, cycling and rumpled eccentricity while nurturing a poorly concealed ambition to lead his country.

Johnson’s decision to break with longtime ally Prime Minister David Cameron and back the “leave” side in Britain’s EU referendum seemed to have paid off when Cameron announced he would resign after last week’s vote in favor of exiting the bloc. The announcement triggered a Conservative leadership race in which Johnson was expected be a front-runner.