Parts of the UK that backed a Leave vote would face the heaviest hit as a result of Brexit, according to estimates by government officials.
The forecasts, seen by MPs, model the 15-year impact of the UK staying in the single market, doing a trade deal with the EU or leaving without a deal.
They suggest that in England, the North East and West Midlands would see the biggest slowdown in growth.
The government said the document did not represent its policy.
It added that the forecasts did not “consider the outcome we are seeking in the negotiations”.
And one Eurosceptic Tory MP said the figures were “complete nonsense”.
Following a leak of some of the information to Buzzfeed last week, and political pressure to release it, ministers agreed to allow MPs to see the reports on a confidential basis in the House of Commons library.
In each scenario in the forecasts, growth would be lower, by 2%, 5% and 8% respectively, than currently forecast over a 15-year period.
In north-east England growth would be 3% lower if the UK stayed in the single market, 11% under a trade deal and 16% with no trade deal compared with staying in the EU, the forecast says.
The research suggests London – which backed Remain – would fare the best, with reductions of 1%, 2% and 2.5% in each of the three scenarios.
Scotland’s estimated hit would be 2.5%, 6% and 9%. Wales would see reductions of 1.5%, 5.5% and 9.5%.
Patrick Minford, of the Economists for Free Trade group, said: “The continued leaks from Whitehall sources about the results of civil servants’ latest modelling attempts is, sadly, a continuation of Project Fear’s effort to paint Brexit as a damage limitation exercise.”
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