A loophole is allowing people who have never paid tax in Britain to bill the NHS for healthcare in their own countries, an investigation has found.
Foreign nationals who claim they are living in the UK are obtaining free European Health Insurance Cards (EHIC) to charge the UK for treatment in their home countries, the Daily Mail said.
The cards are intended for Britons to use to charge the NHS for the cost of any urgent medical treatment they might need while overseas within Europe.
But one was given to an undercover reporter from Hungary who had visited the UK for less than a day, the Mail reported.
The woman went back to her native country where she was told by a number of clinics and hospitals that she could have certain treatments and maternity care paid for on the NHS, the newspaper reported.
The Department of Health (DoH) has insisted there is no evidence of widespread abuse of the system.
But a spokesman admitted the department did not know how many of the five million EHIC cards issued by the NHS each year were given to foreign nationals.
People who do not live in the UK and submit incorrect address details when applying for an EHIC card in Britain are making a fraudulent statement, the spokesman said.
Health minister Alistair Burt said: "It is completely unacceptable that people living outside the UK think they can abuse our NHS.
"That's why this Government has already introduced tough measures to clamp down on migrants using healthcare without making a contribution, to save half a billion pounds within a few years.
"We are continually looking at ways we can tighten up the process further, and as a result of this investigation the Department will urgently carry out more work to include EHIC applications."
The Daily Mail said journalist Ani Horvath - who has never lived or paid taxes in Britain - was able to get an EHIC card and took it to clinics and hospitals in her native Hungary.
They confirmed she could use the card - which lasts five years - to get maternity care and even skin treatments paid for by the UK taxpayer, the newspaper said.
When she asked maternity clinic staff how many Hungarian women had registered for appointments covered by the NHS, she was reportedly told: "A lot of people. More and more."
Using the card, she could have registered for a consultation with an optometrist costing a potential £150, antenatal and birth/ maternity care for one pregnancy at £9,500, or a £47,000 liver transplant, the Mail said.
Consultant surgeon Meirion Thomas told the BBC Radio 4 Today programme it was a well known scam.
He said: "It's already got around, it's there, it's in the real world. Migrants come into this country and ask for EHIC cards.
"Their native EHIC cards they don't have, they're not entitled to (an) EHIC card because they don't make any contribution to the health systems in their country.
"But they all have UK EHIC cards.
"The reason is they can go back to their own country or travel anywhere in Europe and they have got basically health insurance."
The NHS Choices website explains that an EHIC card can enable the holder to get free treatment.
It states: "An EHIC will enable you to access state-provided healthcare in European Economic Area (EEA) countries, including Switzerland, at a reduced cost, or sometimes for free.
"It will cover your treatment until you return to the UK. It also covers treatment of pre-existing medical conditions and routine maternity care, providing the reason for your visit is not specifically to give birth."
To get a card, according to the site, a person must be "ordinarily resident" in the UK, meaning that they normally live there apart from temporary or occasional absences.