BIRMINGHAM, England (Reuters) - Prime Minister Theresa May called on her party on Sunday to unite behind her plan to leave the European Union, making a direct appeal to critics by saying their desire for a free trade deal was at the heart of her Brexit proposals.

Britain's Prime Minister Theresa May walks to the Conservative Party Conference in Birmingham, Britain September 30, 2018. REUTERS/Toby Melville

At the start of what is set to be one of the Conservative Party’s stormiest annual conferences, May’s plans were once again attacked by two former ministers, with her ex-foreign secretary, Boris Johnson, calling them “deranged”.

But she also won strong backing from other Brexit supporting ministers, with trade minister Liam Fox and foreign minister Jeremy Hunt turning their anger against the EU, for “tauntingdeal, a free trade area and frictionless trade ... Chequers at the moment is the only plan on the table that delivers on the Brexit vote ... and also delivers for the people of Northern Ireland.”

May has shown little sign of shifting away from her Chequers plan, named after her country residence where she hashed out an agreement on Brexit with her ministers in July, despite growing criticism that her proposals offer the worst of all worlds.

Johnson, who quit May’s cabinet after Chequers was agreed, called her plans “deranged” and attacked the prime minister for not believing in Brexit.

He, and the former Brexit minister David Davis, are pushing for a Canada-style free trade deal with the EU - a proposal May says will split Northern Ireland from mainland