Boris Johnson's Brexit deal would make every person in the UK £1,100 poorer a year on average, a leading economic forecaster has warned in a damning report.
The National Institute of Economic and Social Research found that the prime minister's withdrawal plan leave the UK £70 billion worse off a year and shave nearly 4% off the economy by the end of the 2020s.
The research comes after MPs voted on Tuesday for a snap general election on December 12, and is one of the first independent assessments of how the economy would perform under Johnson's plan, which he agreed with EU leaders last month.
NIESR estimated that Britain's economy would be 3.5% smaller after 10 years than if it remained in the EU, the equivalent to losing the entire economic output of Wales.
"We don't expect there to be a 'deal dividend' at all," said NIESR economist Arno Hantzsche. "A deal would reduce the risk of a disorderly Brexit outcome but eliminate the possibility of a closer economic relationship."
Unlike the plan agreed by Johnson's predecessor Theresa May, the UK would be likely to leave the EU's customs union after the transition period while Northern Ireland remained signed up to some of its rules.
That would mean tariffs and other barriers would likely represent a significant barrier to trade.
Theresa May's proposed deal would have made Britain's economy 3% smaller than remaining, while a no-deal Brexit would make the country 5.6% poorer, the research found.
The Treasury said it was planning a "more ambitious" trade agreement with the EU than the one NIESR modeled its findings on.
"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," said a spokesperson.
Tom Brake, the Liberal Democrats' Brexit spokesman, said: "It is unconscionable that any Government would voluntarily adopt a policy that would slow economic growth for years to come.
"Boris Johnson's eagerness to push for such a damaging deal is shocking."