In a stark warning, Mr Johnson, who recently resigned from government over the prime minister’s plans for Brexit, will say a “Tory Brexit” label could become an “albatross around our necks for years to come”.
His remarks follow an extraordinary intervention from the chancellor Philip Hammond, who admitted in a radio interview that Britain will be poorer as a result of Brexit as he published new official forecasts.
Every region of the country will be worse off in 15 years’ time than if the UK stayed in the bloc, the analysis said, with London suffering the greatest hit to its economy.
Mr Johnson – the brother of the former foreign secretary Boris Johnson – will say: “The Conservative Party’s reputation for economic competence would be undermined by implementing a botched Brexit, especially one that the government’s own analysis suggests will cause economic harm.”
Referring to Tony Blair’s landslide victory over two decades ago when the Tories lost 178 seats in the Commons, he will add: “Brexit is seen as a project driven by the Conservative Party and this half-baked worst of all words Brexit could trigger an electoral defeat on the scale of 1997 or worse, this ‘Tory Brexit’ label will be an albatross around our necks for years to come.”
“Far from seeking to frustrate the popular will, giving the public the final say would give voice to it at a crucial and fragile time for our democracy.
“That’s why I hope the PM herself will come to see the wisdom of letting the public have the final say. The only way out of this terrible mess is a move to allow the British people to think again about Brexit, and to base their decision on facts rather than fantasy. That is the course that parliament should allow them to take. I urge all my colleagues to give the British people that choice.”
Mr Johnson – appearing alongside Justine Greening, the former education secretary, and Lord Willets – will say of the prime minister’s deal on Thursday: “This is the worst of all worlds. There is a natural opportunity, now that the deal is agreed, for us to check that the country wants us to proceed on this basis.”
At the event organised by the People’s Vote campaign, he will repeat his call for a fresh referendum on the deal and will add: “Now is the time to ensure we have public support for this major decision in our nation’s history.”
But speaking in the Commons on Wednesday at the weekly session of Prime Minister’s Questions, Ms May, who has repeatedly ruled out a fresh vote, said: “It wouldn’t be possible to hold a referendum before 29 March next year – that would mean having to extend Article 50... delaying Brexit or leaving with no deal.”
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