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Boris Johnson's Brexit plan will punch a hole the size of Wales in the UK economy - even if he manages to get a trade deal.

The £70billion-a-year hit is described today in a damning forecast by a respected think tank of the full impact of the Tory leader's deal with the EU.

The National Institute of Economic and Social Research decided to assess the impact after the government refused to do so - claiming it wasn't needed.

Experts said that even if a free trade agreement is agreed with the EU by January 2021, which critics claim is impossible, the UK GDP will be 3.5% smaller each year "in perpetuity" than if we'd stayed in the EU.

"This is roughly equivalent to losing the annual output of Wales," the think tank concluded.

NIESR economist Arno Hantzsche added: "We don't expect there to be a 'deal dividend' at all.

(Image: Getty)

"A deal would reduce the risk of a disorderly Brexit outcome but eliminate the possibility of a closer economic relationship."

Peter Dowd, Labour’s Shadow Chief Secretary to the Treasury, said: “This detailed analysis confirms that Boris Johnson’s deal could leave the UK £70 billion worse off, punching a hole the size of Wales in the UK economy.

“This disastrous deal will result in a huge gap in our public finances, creating a breeding ground for more Tory cuts.

“The fact that Johnson has tried to force this botched deal through shows he does not care about the living standards of millions of people, and cannot be trusted with our economy.”

SNP MP Kirsty Blackman said of the report: " Boris Johnson ’s extreme Tory Brexit would be devastating for Scotland - destroying jobs, and dealing a body blow to living standards, public services and the economy, which would leave the whole country poorer and worse off."

NEISR experts assumed UK-EU goods trade will cut by 40% and services by 60% as Britain leaves the single market and customs union.

"These effects are assumed to build up gradually over time after the end of a transition period in 2021," the NIESR said.

The think tank also assumed a cut in net migration of 50,000 people per year, a 20% cut in foreign direct investment and a 1.3% long-term hit to productivity.

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NEISR was founded in 1938 and claimed to be "Britain's longest established independent research institute" and is not aligned to any political party.

It said: "The economic outlook is clouded by significant economic and political uncertainty and depends critically on the United Kingdom’s trading relationships after Brexit.

"Domestic economic weakness is further amplified by slowing global demand.

"We would not expect economic activity to be boosted by the approval of the government’s proposed Brexit deal. We estimate that, in the long run, the economy would be 3.5% smaller with the deal compared to continued EU membership.

"The economy is estimated to be 2.5% per cent smaller now than it would otherwise have been as a result of the 2016 Brexit vote."

The deal has already been agreed with EU leaders and can be taken up by anyone who becomes Prime Minister after the December 12 election.

If Boris Johnson wins he may decide to push it through Parliament by re-starting his Withdrawal Agreement Bill in the Commons.

But if Jeremy Corbyn wins it could expect to be torn up in favour of a deal that avoids a border in the Irish Sea, enters a customs union with the EU and ensures workers' rights are fully protected.