The agreement of the PM in the UK could leave the economy as much as 5.5% less within a decade than if the UK would remain in the EU, a study said.
And the cost to the public finances could be as much as 1.8% of GDP, the research said by the London School of Economics, King's College and the Institute of Financial Studies.
He blamed the installation of new trade barriers and a fall in immigration.
The government said the agreement was "our national interest".
The report, published by the UK's independent independent thinking tank in Europe, assumes that Britain will continue in a customs union with the EU under the prime minister's approach, but leaves the single market, according to the "backstop" arrangement.
- The Brexit deal will cost UK $ 100 billion & # 39; by 2030
- He can defend the Brexit campaign against MP attacks
However, he said that regulatory barriers to trade in goods and services would increase as a result.
He also stated that the agreement would involve the end of free movement, reducing skilled and non-skilled immigration.
With each other, this would cause GDP per head to be 1.9% -5.5% lower by 2030 than if Britain would remain in the EU – the large range that reflects "significant uncertainty in question".
However, he said that this would increase to 3.5% -8.7% if Brexit did not deal with it.
The study claimed that the reduction in per capita income would be compared in Ireland to the UK, but for most EU countries, it is estimated that the losses are approximately 10 times lower.
He also said that "big economic impact" would have major implications for public finances.
"These would be much more than what won from less EU contributions," he said.
Good to business & # 39;
Professor Anand Menon, director of the UK in Change of Europe said: "Obviously, this type of economic modeling needs to be treated with an appropriate warning.
"However, our estimates give a clear and broad-ranging effect of the effect of the deal discussed by the prime minister."
The study shows a day after research commissioned by the People's Poll, who wants a second referendum, said the PM's Brectic deal would leave the UK of £ 100 billion worse by year by 2030 than it was staying in the EU
A government spokeswoman said: "This agreement will protect jobs and our economy in respect of the outcome of the referendum.
"Provides economic partnership with the EU closer than any other country that enjoys it, is good for business and is of national interest."
First Minister Theresa May will have to persuade MPs in the United Kingdom Parliament to return her deal but she could find it difficult to go.
If MPs refuse the deal, many things could happen – including leaving without dealing, attempting to renegotiate or a general election.