Plans for NHS trusts to charge overseas patients for non-urgent care up front lack detail, the doctors' union has said.
The British Medical Association has warned of possible "chaos and confusion" as a result of the move, announced by Health Secretary Jeremy Hunt, to make it a legal duty for hospitals to check overseas patients for eligibility.
Dr Mark Porter, BMA council chairman, said: "It is right that we ensure all patients are eligible for NHS care and that we have in place a working system to recoup the cost of treatment from patients not ordinarily resident in the UK.
"However, it's hard to see how these new proposals will operate in practice, especially as they are to be implemented by law.
"There is no detail as to how upfront charging will be introduced from scratch in just three months, in an NHS already unable to cope with normal operations.
"We need to be careful not to demonise overseas patients or sow chaos and confusion within the NHS.
"Doctors and nurses cannot be expected to arbitrarily decide whether a patient gets treatment or not.
"There is patchy evidence that this kind of system will achieve £500 million in savings and even if it did, this would not in any way solve the enormous funding crisis in our health service that the Government has for too long ignored.
"Ministers should not mislead the public into thinking this will result in a cash windfall for the NHS, but must address the wider funding shortfall in the NHS, which has left it understaffed and struggling to care for its patients."
Katherine Murphy, chief executive of the Patients Association, said the plans should not compromise patient care.
She said: "We now have NHS Trusts, struggling with funding and capacity issues, seeking to legitimately recoup funds, while at the same time not restrict access to vital emergency treatment.
"The Patients Association believes that as long as the decisions regarding the urgency of treatment are assessed on clinical need, rather than the patient's ability to pay, then the access to care and the wellbeing of the patient should not be compromised.
"As in other countries, if the patient is obviously seeking routine treatment, then it is reasonable that they be told that there is a charge and details of how they are going to pay be taken and immediately acted upon."
Professor J Meirion Thomas, a former cancer surgeon at the Royal Marsden Hospital, who has previously raised concerns about health tourism, said the announcement was a "smokescreen" that would not solve the problem.
He told BBC Radio 4's Today programme: "It's not going to work because there are not trained people to identify and charge these visitors upfront."
Prof Thomas said maternity tourism was a "huge problem" and claimed women were "told in Nigeria to come to this country" to give birth.
Visitors must be made to prove they have health insurance when they apply for a visa, he said.
"That is the cure for this problem in this country but no one will face up to it," he added.