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Theresa May was accused of a “calculated deceit” of the British public by former minister Jo Johnson today.

In his first newspaper interview since resigning on Friday night, Boris Johnson’s brother told the Evening Standard that the Prime Minister was trying to conceal from voters the bitter price of leaving the European Union, even under the terms that she aims to announce this week.

At the same time, Mrs May’s hopes of getting a Brexit deal through Parliament were dented when the leader of Remain-backing MPs, Dominic Grieve, said he was picking up a “sea change” in the mood of back-benchers against the PM’s proposals.

He said more ministers are likely to break ranks, including by making calls for a fresh referendum on whether to press ahead with Brexit or stay in the EU.

From the Brexiteers, there were warnings that Mrs May’s leadership was at stake if she refused to abandon her proposals for a deal that Jacob Rees-Mogg claimed would turn the UK into a “colony of the EU”.

In key developments today:

Deepening Brexit uncertainty caused a steep fall in the pound. In early trading, sterling lost nearly one per cent against the dollar to $1.2845 and was down 0.2 against the euro, at €1.1422.

Penny Mordaunt, the International Development Secretary, fired a warning shot past Mrs May by saying Cabinet could block a bad deal. She said: “The important thing is there’s two checks on this deal, there’s Cabinet and there’s Parliament.”

No 10 was hoping for a decisive breakthrough in talks with the EU in the next 48 hours. Mrs May’s chief Brexit negotiator Olly Robbins was in talks in Brussels with Michel Barnier’s deputy Sabine Weyand aimed at ironing out sticking points and technical issues.

European foreign ministers poured cold water on Mrs May’s hopes of securing a “get-out” clause, demanded by many Tories, to prevent the country being stuck permanently in a Northern Ireland backstop. French Europe minister Nathalie Loiseau said: “If you have any sort of temporary arrangement, this needs to be a bilateral decision from the EU27 and the UK at the same time.”

Jeremy Corbyn was contradicted by his shadow Brexit secretary Sir Keir Starmer, who made clear that the departure from the EU can be “stopped”. Sir Keir said a fresh referendum must be kept as an option.

The four “floating” Cabinet ministers whose verdict will be crucial if a deal is reached — Jeremy Hunt, Sajid Javid, Michael Gove and Dominic Raab — were keeping their powder dry. But a source close to one of them predicted that all four would reluctantly fall into line behind Mrs May because “frankly, there haven’t been enough no-deal preparations to prove an alternative”. Mr Gove, the Environment Secretary, told reporters on his doorstep that any deal must “deliver on the referendum”.

A major pharmaceutical firm announced plans to relocate to Ireland after Brexit. Steris said the move would “preserve benefits” of EU Membership

Unusually, no agenda for tomorrow’s weekly Cabinet meeting was issued to senior ministers before the weekend, suggesting Mrs May was aiming for a breakthrough in time to present a plan for approval.

If talks succeed, a legal text running to 500 pages, setting out withdrawal terms, will be presented. A separate text is expected to outline a political agreement on future trade and relations, with the final deal subject to more negotiations.

In his exclusive interview, former rail minister Mr Johnson said he decided to resign in advance of a deal because leaked plans showed Mrs May would “drown out” the voices of dissenters with a massive public relations campaign to get it through.

He was furious that the leaked plan included telling the public what Mrs May’s deal would mean for the people compared with a shambolic no-deal exit — but would not show how much worse growth would be compared with staying in the EU.

“This is a calculated deceit on the British people,” he said. “I challenge the Government to come clean on the cost of Brexit. The reason they can’t look us in the eye, it’s because they know this will leave us worse off and with less control. It’s a gross abuse of civil service impartiality.”

No 10 rejected Mr Johnson’s claim and disowned the leaked document. “We have made a prior commitment that the House would be provided with appropriate analysis ahead of the meaningful vote” a Downing Street political source said.

Mr Grieve told the Standard that he had found a hardening of opposition to Mrs May’s plans among Tory MPs on both sides of the Brexit debate over the weekend, and more support for a referendum. “It’s clear that we are seeing a deepening crisis,” he said. “The options for a smooth Brexit are non-existent and each day shows this more clearly. We need to consider alternatives, including the public having a final say in a referendum.

“There is a sea-change in mood among my Conservative colleagues who are focused by this crisis. I would not be surprised if more colleagues in senior positions speak out.” Former International Development Secretary Justine Greening said she thought more Remain-backing ministers were considering quitting.

Ms Greening said she did not believe Mrs May’s plan could get a majority in the Commons. “We should be planning as to how we can put this final say on Brexit in the hands of the British people,” she told the Today programme. In his Daily Telegraph column today, Boris Johnson called for the Cabinet to mutiny against plans that would see the UK “remain in captivity”.