A study said the Brexit agreement of PM could leave the economy 5.5% smaller for more than a decade if the UK stayed in the EU.
And the cost of public finance could be up to 1.8% of GDP, said the London School of Economics, King's College and the Institute of Finance Research.
He blamed the imposition of new trade barriers and the decline of immigrants.
The government said the deal was "our national interest".
The UK, an independent think tank, reports in the changing Europe that the UK will remain as a customs union with the EU under the Prime Minister's agreement, but will leave the single market in accordance with the ’backstop ı regulation.
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However, he said it would increase as a result of regulatory barriers to trade of both goods and services.
He also said the agreement would mean the end of free movement by reducing both skilled and unskilled migration.
Taken together, the GDP per capita will be in the EU. If it remained in the EU, it would "reflect" including large uncertainties – "1.9% – 5.5% lower by 2030".
However, he said that if there was no agreement, this would rise to 3.5-8.7%.
The study estimated that per capita income reduction would be comparable to that in the UK but would be about 10 times lower in most EU countries.
In addition, "such a large economic impact" will have great implications for public finance.
”These will outweigh the gains from the reduction of the EU's contribution,“ he said.
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Anand Menon, Director of the United Kingdom in the changing Europe, said: “Obviously, such economic modeling has to be dealt with with appropriate measures.
"However, our estimates are a clear indication of the impact of the agreement negotiated by the prime minister."
The study, a day after the survey conducted by the People's vote to make a second referendum, Prime Minister Brexit agreement by 2030 in the United Kingdom will go to a cut of £ 100 billion.
A government spokesman said: "This agreement will respect the outcome of the referendum and protect our jobs and our economy.
"It establishes a closer economic partnership with the EU than other countries, is good for business and is in our national interests."
Prime Minister Theresa May would now have to convince lawmakers at the United Kingdom Parliament to support the agreement, but could struggle to achieve that.
If MPs reject the agreement, there may be a number of things, such as leaving without a deal, renegotiating, or making general elections.