GPs have too much freedom to register sick foreigners who may not be entitled to expensive British healthcare, campaigners have said.
Migration Watch UK claimed family doctors could decide whether to take on patients without identity documents and give them free treatment which should not be available to them.
The revelation follows a written Parliamentary answer from Health Minister Simon Burns admitting there was no formal requirement for foreigners to provide documentation when registering with a GP.
Mr Burns said: "A decision on whether to register a foreign national who has a six-month visitor's visa is therefore currently for the GP to consider."
Migration Watch chairman Sir Andrew Green said: "What this means is that someone getting off a plane with a valid visitor's visa is in effect able to access the GP services of the NHS without ever having paid a penny into the system.
"Once registered with a GP it is, in practice, an easy step to potentially highly-expensive and long term treatment - all at the expense of the UK taxpayer with little or no prospect of the beneficiaries ever being charged for it."
Sir Andrew said it was not for doctors to "act as an arm of the immigration service" and warned of "clear and substantial risks of abuse in such a lax system".
He added: "The present situation is outrageous. Everyone knows the pressure the NHS is under and its ever increasing cost to the taxpayer. To allow such easy and potentially hugely-expensive access without any entitlement must be stopped at once, otherwise the NHS risks becoming the World Health Service."
Health Minister Simon Burns said: "We won't tolerate abuse of our National Health Service. The NHS has a duty to anyone whose life or long-term health is at immediate risk but it is not there to serve the health needs of the globe.
"There are comprehensive rules and procedures to charge visitors for hospital treatment but we know that the system needs to be improved. That is why we are currently reviewing those arrangements to prevent inappropriate free access to the NHS and provide a fairer, more balanced system."