LONDON (Reuters) - Brexit hardliner Boris Johnson called on Prime Minister Theresa May to rip up her plan for Britain’s departure from the European Union, ratcheting up pressure on May as she prepares to face her divided party at its annual conference next week.

FILE PHOTO: Britain's Prime Minister Theresa May sits next to Britain's Foreign Secretary Boris Johnson as she holds the first Cabinet meeting following the general election at 10 Downing Street, in London June 12, 2017. REUTERS/Leon Neal/Pool/File Photo

The former foreign secretary, who left the government over his opposition to May’s “Chequers” proposals, later on Friday swerved the question of whether he might challenge her leadership.

Just six months before Britain is due to leave the European Union on March 29, 2019, little is clear: May has yet to clinch a Brexit divorce deal with the EU and rebels in her party have threatened to vote down any deal she makes.

Adding to the uncertainty, a poll of polls published on Friday showed voters would now vote 52 to 48 percent in favor of remaining in the EU were there to be another Brexit referendum. May has repeatedly ruled out another referendum.

Johnson, the bookmakers’ favorite to succeed May, said her Brexit plans would leave the United Kingdom half in and half out of the club it joined in 1973 and in effective “enforced vassalage”.

“This is the moment to change the course of the negotiations and do justice to the ambitions and potential of Brexit,” Johnson wrote in Friday’s Daily Telegraph.

Under the headline, “My plan for a better Brexit”, Johnson, called for a “SuperCanada-type free trade agreement”. He said the EU’s “backstop” proposals for Northern Ireland, under which the British-ruled province would remain within the EU customs union even if the rest of Britain left, amounted to the economic annexation of part of the United Kingdom.

Johnson later declined to answer directly when asked if he would rule out a leadership challenge of the Conservative party.

“My job is to speak up for what I believe in and the vision that I have set out today and I believe in it very, very sincerely and I’m going to keep going for as long as it takes,” he told the BBC’s political editor Laura Kuenssberg.

FRINGE SPEECH

May, who voted to stay in the EU, is trying to clinch a divorce deal with the EU while grappling with an open rebellion in her Conservative Party, which convenes in the English city of Birmingham on Sunday for its annual party conference.

May has repeatedly said her Brexit proposals are the only viable ones.

Boris Johnson is scheduled to speak at 13.00 on Tuesday at a fringe event at the conference.

More than two years since the 2016 Brexit referendum, the United Kingdom, its politicians and its business leaders remain deeply divided over Brexit, considered to be one the most important decisions in post-World War Two British history.

In the June 23, 2016 referendum, 17.4 million voters, or 51.9 percent, backed leaving the EU, while 16.1 million voters, or 48.1 percent, backed staying. Some recent surveys have shown a swing toward support for staying in the EU.

If there were to be another referendum, researchers said a narrow victory for those hoping to reverse Brexit would be heavily contingent on getting those who did not vote last time to turn out.

“True, Remain enjoys a lead in the polls. But that lead remains a narrow one, and there is little sign of it growing,” said John Curtice, Britain’s most prominent polling expert.