Nearly nine in 10 European Union workers in the UK would fail to meet visa requirements and would face losing their jobs in the event of Brexit, presenting a "serious challenge" to thousands of firms, a report has found.
The research found only 12% of the 1.6 million EU citizens working in the UK would meet visa requirements under the policies applied to people from the rest of the world.
Although almost 14% of workers in the accommodation and food sectors are from the EU, less than 1% would qualify under the UK visa policy, the research by the Social Market Foundation (SMF) think tank and recruitment agency Adecco found.
Remain campaign Britain Stronger In Europe said the impact of Brexit and the end of EU free movement rules would be "catastrophic" for public services which rely on European workers.
Nigel Keohane, co-author of the SMF report, said: "Our research shows the reliance that UK employers have on EU workers.
"In the event of a vote to leave we do not know what immigration regime would take the place of free movement.
"Our analysis of the potential effects of applying the current visa policy for non-European workers to EU workers and other potential strategies reveals the potential scale of the effect on UK employers."
Adecco's managing director Adam Hawkins said: "Under a scenario where free movement of labour no longer applies and EU workers were subjected to the same visa requirements that are currently in place for non-EEA workers, 88% of EU workers currently working in the UK would fail to qualify.
"Filling this gap may pose a real challenge for UK employers.
"With 1.6 million EU workers currently making up around 6% of the UK workforce, it is clear that a severe change to our relationship with the European Union could provide a serious challenge to thousands of British businesses."
Immigration is one of the key battlegrounds in the Brexit debate, with Leave campaigners claiming the number coming to the UK is unsustainable and EU incomers have hit job chances and pay rates for UK workers.
Around 270,000 EU citizens came to the UK last year - up 6,000 - with 86,000 Brits heading the other way, giving a net migration figure of 184,000, 10,000 more than the previous 12 months.
But the Remain camp claim immigrants have been economically beneficial and play a vital role in public services such as the NHS.
Office for National Statistics figures show 240,000 EU citizens work in the public sector, including more than 130,000 on health and social care roles.
Shadow Treasury chief secretary Seem Malhotra said: "Leave campaigners like Boris Johnson and Nigel Farage have spent years arguing for cuts in the public sector, including our NHS. Now they want to burden vital services with staff shortages too.
"Leaving Europe would be catastrophic for the public sector, leaving services under-manned and staff overstretched.
"Anyone who works in or depends on our public service should reject the divisive, dog-whistle Leave campaign and vote to remain in Europe on June 23."
Unison general secretary Dave Prentis said: "If we left the EU, our NHS and social care could quickly be at the point of collapse as, at a stroke, it would be much harder to recruit nurses, midwives, and other health workers.
"Spending on agency workers would go through the roof as hospitals and councils were forced to look elsewhere for staff.
"Workers from across Europe are making a valuable contribution to Britain, helping to keep us all healthier, safer and well cared for.
"The economic turmoil that would result from a vote to leave the EU would be a hammer blow to our public services, which would leave patients, staff and service users much worse off."
Brexit-backing Tory MEP Daniel Hannan said: "The In campaign are trying to distract from the out of control immigration system that we are locked into while we stay in the EU.
"If we vote Leave we will be able to take back control of our borders and create a fairer immigration system that prioritises the skills that we need here in the UK and stops the open door policy which puts pressure on public services."