Johnson quit the government in July, citing his opposition to the prime minister's plan for close regulatory and economic ties with the European Union after Britain's departure. He told the BBC in an interview Friday that May's proposal "doesn't take back control it relinquishes control. It forfeits control" because it means accepting EU rules.
Johnson advocates a looser "Canada-style" agreement that would leave Britain freer to strike new international trade deals. With six months to go until Britain leaves the EU on March 29, negotiations on the terms of the divorce are at a standstill. Britain's governing Conservatives are deeply divided over how to proceed.
Johnson declined to rule out a challenge to May's leadership when asked about it repeatedly. But he told the BBC said the prime minister is "a remarkable person. She will go on for as long as she feels it necessary."
Johnson is due to speak at a Conservative Party convention next week, a day before May's scheduled keynote address to delegates. The flamboyant Johnson made sure to be visible ahead of the gathering, giving television interviews and writing a 4,500-word Daily Telegraph article branding May's plan "a moral and intellectual humiliation" for Britain.
But the loquacious politician was cagey about his leadership ambitions. Asked whether Britain needed a change of leadership, he told Sky News: "It's not about that, it's about the direction that we're going in."
He told Sky that if May's government changed course and embraced the looser relationship with the EU that he advocated, "my cup will run over, it really will."