The government's latest analysis of Brexit's impact concluded the UK will be worse off under every scenario modelled, according to reports.
Entitled "EU Exit Analysis – Cross Whitehall Briefing”, the draft assessment was dated this month.
Reported by Buzzfeed, under a comprehensive free trade agreement with the EU economic growth would be five per cent lower over the next 15 years compared with current forecasts, the analysis concluded.
A "no deal" Brexit – with Britain reverting to World Trade Organisation (WTO) rules – would see growth eight per cent lower over the same period. Continued access to the single-market by way of membership of the European Economic Area would be comparatively better, with growth two per cent lower.
Each scenario assumes the UK will strike a trade deal with the US.
Short-term shocks to the UK economy from adjusting to a new customs arrangement were not included in the assessment.
The tightly-guarded report, prepared by Whitehall officials for the Department for Exiting the European Union, is due to be presented to key ministers at one-to-one meetings this week, the Times reported today.
Government sources told Buzzfeed the findings would not be made public on account of them being "embarrassing".
Anti-Brexit campaigners said the findings proved Brexit was "a colossal act of economic self-harm".
"We are reading about an economy facing the abyss," said Best for Britain chief executive Eloise Todd.
However, influential Brexiteer MP Jacob Rees-Mogg responded to the report by questioning the validity of the research behind the draft papers.
"If they use the same type of gravity models the Treasury used prior to [the Brexit vote], they were fundamentally wrong," he tweeted.
A government spokesperson said: "We have already set out that the Government is undertaking a wide range of ongoing analysis in support of our EU exit negotiations and preparations.
"We have been clear that we are not prepared to provide a running commentary on any aspect of this ongoing internal work."