The UK agreed that, in the absence of an overall deal, it will continue to fully align with the rules of the customs union and single market that are necessary for cross-border co-operation and the protection of the all-island economy.
They suggest that in England, the North East and West Midlands would see the biggest slowdown in growth. The losses are predicted to come over 15 years.
No deal with the European Union will leave North East with a 16% dip and the West Midlands with a 13% hit to growth. By contrast, London would only be affected by 3.5% under the worst-case scenario. Wales would see reductions of 1.5%, 5.5% and 9.5%.
Brexit-backing Conservative MP Jacob Rees-Mogg accused Treasury officials of "fiddling the figures" to make all options but staying in the European Union look bad.
The North East is in line for a 16 per cent hit in GDP in the eventuality of a Brexit no deal, government analysis has revealed in a new leak.
The government has said the analysis is preliminary and crucially does not measure the impact of the UK's preferred option of a bespoke and comprehensive trade agreement, covering goods and financial services.
By comparison, London would sustain just a 2 per cent hit to growth if the United Kingdom gets a free trade deal, 3.5 per cent in a no deal scenario, and just 1 per cent if the country stays in the single market.
SNP MP Peter Grant said: "The leaked United Kingdom government's internal Brexit analysis makes for utterly grim reading, and Theresa May must now end the shameful secrecy surrounding the United Kingdom government's Brexit plans and publish the papers in full - which we now know will have catastrophic consequences for Scotland and the UK's economy".
Northern Ireland would suffer an 8 per cent dip in growth after a free trade deal, a 12 per cent dip in the event of "no deal", and a 2.5 per cent fall in the single market.
BREXIT could cause the north's economy to shrink by up to 12%, according to British government figures.
A free trade deal would result in a 8% hit to growth in the West Midlands, compared with 13% under a no deal.
The figures emerged as representatives of Nissan and other Japanese companies are set to meet Theresa May and Chancellor Philip Hammond on Thursday.
"As the poorest English region, the North East was inevitably going to be more vulnerable to the economic shock of leaving the EU".
He added: "I'm sure they [the United Kingdom government] won't want to renege on an agreement they only made a few months ago, and they would find it very hard to make any future agreement with Ireland, the European Union or anyone else in the world if they tried to depart from an agreement they made a few months ago".
"We are not into talks about trade yet, because we think it's important that we prioritise the withdrawal agreement and transition phase and make sure what was agreed in December is written into the legal text of that agreement", he added.