A top health think tank has warned cracking down on immigration could slash the number of health and care staff in the UK.
The highly respected Nuffield Trust issued a warning to political parties to "tread carefully" - just weeks after the Tories announced a crackdown on people coming to Britain.
Boris Johnson has vowed to control EU migrants' access to the UK with a points-based immigration system.
Health workers would get a special "NHS visa" - but the Nuffield Trust today warns the details "remain completely unclear", meaning it's "impossible to tell whether it will be more or less open" than the current system.
And the Tories are also planning to slap EU migrants with a newly-raised, £625-a-year Immigration Health Surchage to use the NHS - even if they work in it. It's been dubbed a 'nurse tax'.
Meanwhile, the Trust says Labour has a "positive" plan to exempt those recruited by health trusts - but also warns the manifesto is "ambiguous" and previous similar changes led to a slowdown.
It came as separate analysis by the Health Foundation found the Tories' multi-billion pound pledges for the NHS would not be enough to improve care.
And on social care, the Tories' £1.1bn-a-year sticking plaster will not even do a third of the job of stabilising the current system.
Anita Charlesworth, Director of Research and Economics at the Health Foundation, said: "All three parties have pledged some funding towards addressing the current crisis in social care and preventing further deterioration of services, over and above current plans for 2023/24.
"However, none have pledged enough to meet growing levels of demand and improve pay for social care staff."
The Nuffield Trust says there is an "obvious concern" that either main party's plans could trigger a repeat of previous trends which saw around 6,000 fewer health staff coming to the UK per year.
The analysis says around a quarter of UK hospital staff were born abroad and immigration has helped stop "almost unimaginable" shortages in the NHS workforce.
Around half of the increase in the health and social care workforce over the last decade has been from workers born outside the UK, according to figures from the Office for National Statistics.
The Nuffield Trust, which requested the data, said the number of staff grew by 446,000 between 2009/10 and 2018/19, with 221,000 of these workers born overseas.
People born abroad made up almost a fifth (818,000) of private and NHS staff in the health and social care workforce in 2018/19, compared to 14% of the general population.
And reliance was greatest in hospitals, where 23% (324,000) of workers were born outside the UK.
The think tank is calling on the main parties to "tread carefully with their migration policies" in its new briefing: Stopping the staff we need? Migration choices and the 2019 election.
Mark Dayan, Nuffield Trust policy analyst, said: "This analysis reveals just how international the NHS truly is, and that without migration staffing shortages would be almost unimaginable.
"The Conservatives and Labour have made encouraging assurances to enable some foreign NHS staff to arrive after we leave the EU.
"But these pledges will fall flat if not matched with promises to recruit social care staff from abroad and expanded to other vital NHS staff beyond hospital nurses and doctors.
"With the NHS continuing to be a top priority for voters, restricting migration could backfire spectacularly given we already have dire shortages and more staff are desperately needed".
