The government's secret Brexit impact analysis shows that the cost to the British economy of cutting migration from the EU would be significantly greater than the benefits brought by a US trade deal.

BuzzFeed News revealed on Monday that the assessment, compiled by officials across Whitehall for the Department for Exiting the European Union (DExEU), said UK economic growth would be lower under all three main Brexit scenarios that were modelled.

The analysis also includes several potential immigration policies to illustrate the impact falling EU migration after Brexit would have on the economy.

It shows that the negative impact of a stricter immigration policy – replacing free movement with arrangements in line with those for non-EU citizens – would be far bigger than the 0.2% boost to economic growth that a US trade deal is calculated to bring.

Even a more flexible immigration policy, which would lead to a smaller drop in migration from the EU, would still be enough by itself to cancel out the benefits of a US deal, the document shows.

The findings starkly illustrate the difficulties Theresa May faces as she attempts to shape a Brexit deal that minimises the long-term hit on the economy while satisfying the demands of Leave voters.

Immigration was front and centre of the Vote Leave campaign in 2016’s referendum, and was one of the key issues driving the Brexit vote, according to data from the British Election Study.

Freedom of movement will continue in effect for the two-year transition period that is currently being negotiated with the EU. The UK will then introduce a new immigration regime, though the government has yet to spell out what this would look like.