Britain will be worse off outside the EU whatever kind of Brexit deal it strikes with Brussels, the latest Government impact assessment is reported to have found.
The analysis, prepared for the Department for Exiting the EU (DexEU), concluded growth would be lower under a series of potential scenarios, according to a leaked copy seen by BuzzFeed News.
Even if the UK is able to negotiate a comprehensive free trade agreement – as Theresa May hopes – it estimates that growth will be down 5% over the next 15 years.
That would rise to 8% if Britain left without a deal and was forced to fall back on World Trade Organisation (WTO) rules.
Alternatively, if the UK were to retain access to the single market through membership of the European Economic Area the loss would be just 2%.
The leaked findings are likely to infuriate pro-Brexit Tory MPs who already suspect the Government is heading for a “soft” break which will retain many of the existing elements of the relationship with Brussels.
A Government spokesman 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 and that ministers have a duty not to publish anything that could risk exposing our negotiation position.”
A Government source added: “As part of its preparations for leaving the European Union, officials from across Whitehall are undertaking a wide range of ongoing analysis.
“An early draft of this next stage of analysis has looked at different off-the-shelf arrangements that currently exist as well as other external estimates.
“It does not, however, set out or measure the details of our desired outcome – a new deep and special partnership with the EU – or predict the conclusions of the negotiations.
“It also contains a significant number of caveats and is hugely dependant on a wide range of assumptions which demonstrate that significantly more work needs to be carried out to make use of this analysis and draw out conclusions.”
Former Tory cabinet minister and leading Brexiteer Iain Duncan Smith said the analysis was “deliberately leaked” because it “gives a bad view.
Speaking to BBC Radio 4′s Today programme, Duncan Smith said the leak was “a little bit suspicious” and dismissed the analysis as based on government economic models that were “highly discredited”.
“It’s deliberately leaked because it gives a bad view, we should put it on one side and leave it alone,” he said. “It’s an incomplete report.”
He said if the government got Brexit “right” then the “economic will grow, food prices will come down and particularly the poorest in society will benefit”.
Duncan Smith added: “None of is know exactly where this will end up.”
However Labour MP Chris Leslie, a member of the Open Britain group which campaigns against a “hard” Brexit, said ministers must now release the findings in full.
“No one voted to make themselves or their families worse off,” he said. “The Government must now publish their analysis in full, so that MPs and the public can see for themselves the impact that Brexit will have and judge for themselves whether it is the right thing for our country.”