Overseas patients have left the NHS with unpaid bill of almost £30 million in just one year - including £100,000 owed for healthcare in Sheffield.
Patients who were not entitled to free treatment on the health service owed £29, 530,378 last year, according to figure obtained under the Freedom of Information Act.
More than 120 NHS trusts were asked to provide details on overseas patients who were billed for NHS care but failed to pay.
Some 104 trusts across England responded, with the results suggesting the unpaid bill is rising year on year.
Some trusts are still trying to chase the money from overseas patients while others have written a portion off as bad debt.
The true amount owed is likely to be far higher as the figures only relates to those people identified as being chargeable. Many other overseas patients, including those who are able to
Eight hospital trusts in London are owed more than £1 million and a further 21 across England are owed at least £100,000, including hospitals in Sheffield, Liverpool, Birmingham and Oxford.
Katherine Murphy, chief executive of the Patients Association, said: "It is shocking that so many costs are left unpaid by overseas patients using the NHS. If this money was reinvested in the NHS it could mean the difference between hiring more nurses or paying for additional equipment.
"Patients have very strong feelings about overseas visitors, and understandably, patients feel that an NHS with diminishing resources should be prioritising UK citizens first and foremost."
Dr Mark Porter, chairman of the British Medical Association, said: "It is important that costs are recouped from patients who are not eligible for NHS treatment, but systems 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.
"A doctor's duty is to treat the patient in front of them, not to act as a border guard.
"Sick and vulnerable patients must not be deterred from seeking necessary treatment, otherwise there may be serious consequences for their health and that of the public in general."
John O'Connell, chief executive of the TaxPayers' Alliance, said: "Barely a day goes past without health bosses pleading poverty and asking for more money yet these are huge sums that are just being written off.
"Obviously those in need of emergency care while in Britain should get it, but our free at the point of use health system is far more open to abuse than those of continental Europe.
"What's more, these figures represent only what the NHS knows it's owed - much abuse of the system goes unidentified and best estimates of the true cost put it 10 times higher.
"The Government has stressed the importance of recouping the money they can from chargeable patients but if this is not happening or if health bosses are not bothering to do it, the Government will have to start penalising those that are failing in their duty to taxpayers."