Less damaging exit terms, under which Britain would secure a free trade agreement with the rest of the EU, would still result in national output being £131bn lower over the same period.
The Best for Britain Group, which is campaigning to halt Brexit, said its research – based on the Government’s own leaked analysis – fully exposed the “Brexit black hole at heart of the economy”.
“Sadly, now we are seeing the economic analysis becoming project fact and it means that we are facing a massive Brexit blow,” said Lord Mark Malloch-Brown, the organisation’s chairman.
“Losing billions means we will have less cash for the NHS, our schools and our public services. This all shows the best option for communities up and down the country is to stay in and keep the deal we have.”
The calculation is the most comprehensive attempt yet to put a price on the economic damage forecast in the Treasury Brexit analysis which the Government was – eventually – forced to release to MPs.
It predicted a no-deal Brexit, leaving Britain trading with Europe on World Trade Organisation terms, would reduce growth by 8 per cent over 15 years.