The assessment, which is titled “EU Exit Analysis – Cross Whitehall Briefing” and dated January 2018, looked at three of the most plausible Brexit scenarios based on existing EU arrangements.

Under a comprehensive free trade agreement with the EU, UK growth would be 5% lower over the next 15 years compared to current forecasts, according to the analysis.

The "no deal" scenario, which would see the UK revert to World Trade Organization (WTO) rules, would reduce growth by 8% over that period. The softest Brexit option of continued single-market access through membership of the European Economic Area would, in the longer term, still lower growth by 2%.

These calculations do not take into account any short-term hits to the economy from Brexit, such as the cost of adjusting the economy to new customs arrangements.

The assessment seen by BuzzFeed News is being kept tightly guarded inside government. It was prepared by officials across Whitehall for the Department for Exiting the European Union (DExEU) and is reportedly being presented to key ministers in one-to-one meetings this week ahead of discussion at the Brexit cabinet subcommittee next week. [...]

Even though the analysis assumes that the UK will agree a trade deal with the US, roll over dozens of the EU’s current trade agreements, and consider loosening regulations after Brexit, there is no scenario that does not leave the country worse off.

Officials believe the methodology for the new assessment is better than that used for similar analyses before the referendum.

The January 2018 analysis looked only at existing EU arrangements, which means bespoke arrangements have yet to be modelled. Prime Minister Theresa May has repeatedly said she is seeking a "deep and special partnership with the EU".

The other main findings of the analysis:

• Almost every sector of the economy included in the analysis would be negatively impacted in all three scenarios, with chemicals, clothing, manufacturing, food and drink, and cars and retail the hardest hit. The analysis found that only the agriculture sector under the WTO scenario would not be adversely affected.

• Every UK region would also be affected negatively in all the modelled scenarios, with the North East, the West Midlands, and Northern Ireland (before even considering the possibility of a hard border) facing the biggest falls in economic performance.

• There is a risk that London’s status as a financial centre could be severely eroded, with the possibilities available under an FTA not much different to those in the WTO option.

• On the plus side, the analysis assumes in all scenarios that a trade deal with the US will be concluded, and that it would benefit GDP by about 0.2% in the long term. Trade deals with other non-EU countries and blocs, such as China, India, Australia, the Gulf countries, and the nations of Southeast Asia would add, in total, a further 0.1% to 0.4% to GDP over the long term. [...]

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