Britain Foreign Secretary, Boris Johnson

UK former Foreign Minister Boris Johnson has attacked Theresa May’s Brexit plan at an event putting the economic case for leaving the EU without an agreement on trade.

The Economists For Free Trade report said the UK had “nothing to fear” from a “clean break” from the EU and using World Trade Organisation rules.

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This could give a £80bn boost to the tax base and cut prices by 8%, it said.

But the claims were branded “Project Fantasy” by Labour MP Chuka Umunna.

And Chancellor Philip Hammond said the economic assumptions behind the analysis were “not sustainable” and out of line with other forecasts.

Hammond, who earlier on Tuesday announced Bank of England Governor Mark Carney would be extending his contract until January 2020 to provide continuity after Brexit, has issued a fresh warning of “some turbulence” if the UK left the EU in March without a deal.

Ministers have said reaching a general agreement on future economic co-operation with the EU is in the UK’s interests before it leaves on 29 March 2019.

The UK and EU are hoping to agree on a way forward next month, at the same time as they settle the terms of the UK’s withdrawal from the bloc, including a transition period up to the end of 2020.

Johnson and other Conservative MPs opposed to Theresa May’s Chequers plan for future relations with the EU are facing growing pressure to provide an alternative plan.

Johnson, who made a surprise appearance at the launch of the report in Westminster, said May’s Chequers plan would mean “abandoning our seat around the table in Brussels and continuing to accept the single market legislation”.

He added: “That seems to me to be a particular economic risk in Chequers and makes it substantially worse than the status quo.”