LONDON • Mrs Theresa May battled to assert her authority as British prime minister yesterday after a disastrous start to her Conservative Party's annual conference threatened to explode into a full-blown leadership crisis.

Before the event had even begun, Mrs May - who became prime minister in the aftermath of Britain's 2016 EU referendum - found herself fending off a challenge over Brexit from former foreign secretary Boris Johnson.

Mr Johnson told the Sunday Times that Mrs May's Brexit proposal was "deranged" and attacked her for not believing in Brexit.

Together with former Brexit minister David Davis, the two are pushing for a Canada-style free trade deal with the European Union - a proposal Mrs May says will split Northern Ireland from mainland Britain by making the British province adhere to different Customs rules.

Mr Davis, who like Mr Johnson resigned in protest over her plan, said her proposal was "just wrong", but added that he thought it was 80 to 90 per cent likely that the government would strike an exit deal with the EU.

Mrs May refused to be drawn on his comments, but issued a sharp response. "I do believe in Brexit," she told the BBC.

"But crucially, I believe in delivering Brexit in a way that respects the vote and delivers on the vote of the British people while also protecting our union, protecting jobs and ensuring that we make a success of Brexit for the future."

She insisted that her proposed blueprint for leaving the EU was not dead and appealed to her critics to put the needs of the country first.

Adding to Mrs May's woes, the party was yesterday investigating a data breach which let members of the public log into a smartphone app as senior government ministers and view their personal details.

Meanwhile, the government said it would launch a consultation to increase stamp duty on individuals and companies not paying tax in the United Kingdom. Ministers are considering a rate ranging from 1 per cent to 3 per cent, according to the Sunday Telegraph newspaper.

"Currently, foreign buyers can purchase homes in the UK as easily as people who live here, but there is evidence this is inflating house prices," the government said.

Mr Henry Pryor, a real estate broker, said: "This policy, with its uncomfortable echoes of blaming foreigners for every ill, may make good headlines, but it sends an uncomfortable message to the rest of the world and will do nothing to create more homes for those unable to buy or to rent today."

BLOOMBERG, REUTERS