Boris Johnson’s Brexit deal ‘would deliver £70bn hit to economy by 2029’

That would lead to the economy being 3.5% smaller, the NIESR found.

A no-deal Brexit would be worse though – knocking 5.6% off the expected size of the economy.

NIESR economist Arno Hantzsche said: “We don’t expect there to be a ‘deal dividend’ at all.

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

Opposition parties seized on the analysis though the Treasury said the UK was aiming to secure a “more ambitious” trade deal than that on which NISER’s findings were based.

Liberal Democrat Brexit spokesman Tom Brake said: “The Tories’ obsession with Brexit at any cost puts our future prosperity at risk.

“It is unconscionable that any government would voluntarily adopt a policy that would slow economic growth for years to come.”

A Treasury spokesman said: “A Brexit deal will give people and businesses the certainty they need to invest, which will support our economy.

“We are aiming to negotiate a comprehensive free trade agreement with the European Union, which is more ambitious than the standard free trade deal that NIESR has based its findings on.”