The price of NHS treatment for tens of thousands of British pensioners returning to the UK from Spain, France and other EU countries after Brexit will hit a billion pounds, experts have warned.

Shortages of NHS and social care staff and extra charges for new drugs are likely to hike costs for the health service even higher when Britain leaves the EU, according to a new report from health think tank the Nuffield Trust.

The Department of Health currently spends around £500m on a scheme that allows some 190,000 pensioners to access free or reduced-cost medical treatment in EU countries.

However, it is unlikely this reciprocal arrangement will be kept after Brexit, meaning the NHS will face a bill of almost £1bn in total – double the current outlay. There will also be severe pressure on hospital beds, as the health service struggles to cope with the extra patients, and a shortage of staff, said researcher Mark Dayan.

Mr Dayan told The Independent the situation for the NHS after Brexit will be “difficult”, adding that the total cost faced by the health service could be even higher if Brexit causes an economic slowdown that impacts on public finances.

A bigger problem than the costs is "the need for additional staff and hospital beds”, he said. “You can’t just turn on a tap and produce these things. They’re limited resources and are already overstretched in the NHS.”

“The impact of staffing shortages, which we already have and could worsen after Brexit if handled badly, are that some places won’t have enough staff to operate safely, or agency staff will have to be brought in at high rate, which will make the NHS’s financial position even worse.”

If the NHS needed to care for those who currently receive care abroad, it would need a significantly higher number of hospital beds – around 900, or the equivalent of two new hospitals, said the report.

Brexit negotiators should try to secure a deal that would mean that expats still receive care in the country they reside in, it added.

The report comes as the Institute for Fiscal Studies warns the NHS faces an “incredibly challenging” five years after the general election on 8 June.

All three major political parties have promised to increase health spending, but all the proposals fall well short of the 4 per cent average annual increase in health spending, fuelled by Britain's growing and ageing population, the increasing prevalence of chronic conditions and the development of expensive new technologies and drugs.

Jean McHale, a professor of health care law at the University of Birmingham, said it is in Britain’s best interest to treat expats in the country they live in, as it is more expensive to treat them under the NHS.

Currently 70,000 retired UK citizens use Spain’s health system. Sue Wilson, head of the campaign group Bremain in Spain, said those on low incomes may have no choice but to return to the UK for healthcare after Brexit.

“The biggest fear for people here, many who have been here for a long time, is that they would be forced to go home against their wishes because of the finances," she told The Independent.

"It’s not a decision they would gladly make, but if they couldn’t afford healthcare here, they would have to go back to get healthcare from the NHS."

“Your only option is to buy into the Spanish system, which costs €157 (£137) a month. On top of that you’ve got to pay for your prescriptions, 100 per cent rather than the 10 per cent they pay at the moment.

“Private healthcare is very expensive in Spain and people with pre-existing conditions aren’t eligible. If they are on a low income to start with, €157 out of a meagre pension is a lot of money, especially if they have to pay for drugs on top.”

The report suggested that social care faces a shortfall of as many as 70,000 workers by 2025-26 if net migration from the EU is halted after Brexit.

In April, a leaked internal document from the Department of Health predicted a worst-case scenario of a possible shortage of up to 42,000 nurses.

The British Medical Association (BMA) echoed the think tank’s calls for the Government to grant permanent residence to doctors from the EEA working in the UK.

“These figures are a stark reminder that with the NHS at breaking point, politicians must keep the health service and its patients at the forefront during Brexit negotiations and reduce the impact that leaving the EU will have on health and social care across the UK,” said Dr Mark Porter, the union’s council chair.

“Not only might NHS resources fall, but existing chronic staff shortages could be worsened as half of the 10,000 EEA doctors working in the NHS are considering leaving the UK. This would seriously impact patient care across the country and increase what are often already unacceptable delays for treatment.”

Tim Farron, leader of the Liberal Democrats, said the "stark figures show Theresa May's extreme version of Brexit would be a disaster for the NHS.

"Crashing out of the EU without a deal would mean the loss of healthcare rights for British pensioners in Europe, putting huge pressure on our hospitals. We risk seeing nurses and social care workers from the EU leaving in their droves because Theresa May won't do the right thing and guarantee their right to stay".

A Conservative party spokesman said the party has been "clear that safeguarding the rights of UK nationals in the EU and EU nationals in the UK is one of our first priorities for the Brexit negotiations which will begin just 11 days after polling day."