Doctors have warned they must not be expected to act as border guards after it emerged that charges for emergency healthcare are set to be imposed on patients from overseas.
Health Secretary Jeremy Hunt wants to extend the fees paid by people from outside the European Union for planned hospital care to those who turn up at accident and emergency departments.
Officials have insisted that no one who needs urgent care will be turned away, with systems put in place to recoup the costs after treatment.
But patients needing other casualty department services face being forced to pay upfront before they are treated.
The British Medical Association, which represents doctors, has raised concerns about the plans.
A spokesman said: "A doctor's duty is to treat the patient in front of them, not to act as a border guard.
"Any plans to charge migrants and short-term visitors need to be practical, economic and efficient and must not jeopardise access to healthcare for those who need it."
The crackdown on so-called health tourism is expected to claw back around £500 million, according to The Times.
Plans are expected to go out to consultation in the next few weeks.
Exemptions will be put in place for refugees and asylum seekers, and pregnant women will not be turned away from maternity units if they have not paid upfront under the proposals.
A Department of Health spokesman said: "International visitors are welcome to use the NHS, provided they pay for it - just as families living in the UK do through their taxes.
"This Government was the first to introduce tough measures to clamp down on migrants accessing NHS care and have always been clear we want to look at extending charges for non-EEA migrants.
"No one will be denied urgent treatment and vulnerable groups will continue to be exempt from charging."