Between 50,000 and 250,000 British expats currently living in the European Union may return to the UK in the event of a no-deal Brexit, potentially putting significant pressure on already-stretched public services, according to government assessments shared with BuzzFeed News.

An internal memo circulated around Whitehall departments revealed that officials are preparing for a sudden rush of citizens to return home if Britain crashes out of the EU without a negotiated settlement, including many pensioners who had retired to the continent.

Under the “worst case scenario” envisaged in the cross-Whitehall contingency planning document, about 150,000 people would return in the first year after the UK leaves the EU on March 29 — with around 30,000 coming back in the first three months.

Another 100,000 would follow in the next wave of the “Brexodus” the following year, according to the estimate.

The Department for Work and Pensions has a more optimistic assessment of the likely influx and is projecting that around 50,000 expats would return home if there is no deal.

Around 1.3 million British citizens reside in countries across the EU, and there are concerns that an abrupt severing of the UK’s relationship with Brussels would badly disrupt their lives, affecting things such as their ability to work in their adopted countries and to access pensions and savings back in the UK.

A source familiar with the government’s thinking told BuzzFeed News that the “central planning assumption” was that around 40% of the returning expats would be pensioners, placing fresh strains on the National Health Service and social care services that are already struggling.

Officials at the DWP are calculating that the vast majority of expats will stay where they are after assurances from Spain and other EU governments that their rights will be respected even if the UK crashes out without a deal.

A government spokesperson said these assurances meant the worst-case scenario was “misleading” and out of date. A DWP source said the number of returning Brits would be closer to 50,000.

It is normal for the government to plan for all possible contingencies, including a sudden spike in repatriations.

There have also been concerns that British citizens moving back to the UK could fall victim to the Home Office’s “hostile environment” policy if they try to claim benefits, another Whitehall source said.

“Some of these people, if they were born abroad to British parents, might not have ever stepped foot in the UK, so there is going to be very little record of them,” the source said.

A government spokesperson said: “We are looking at options to see what more we can do to ensure that any returning UK nationals get immediate access to the benefits and support they are entitled to.”

There have already been signs that expats have been heading back to the UK since the Brexit vote.

The number of British expats in Spain fell by almost 8,000 to 285,698 in 2017, according to the country’s National Statistics Institute.

In the last five years, the number of British residents in Spain has fallen by 157,107.