Overseas patients have left the NHS with unpaid bill of almost £30 million in just one year, an investigation has found.
Patients who were not entitled to free treatment on the health service owed £29,530,378 in 2015/16, according to data obtained by the Press Association 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 and who were exempt from reciprocal arrangements with the UK.
Some 104 trusts across England responded, with the results suggesting the unpaid bill is rising year on year. Of 92 trusts that were also able to give data for 2014/15, the outstanding debt in that year came to £15,907,018.
Several NHS trusts provided figures for the last four years, showing they are now owed a backlog of millions of pounds.
In London, Barts is owed £10,340,322 from the last four years, while Imperial College Healthcare NHS Trust is owed £5,509,733.
Guy's & St Thomas' NHS Foundation Trust and London North West Healthcare NHS Trust are also owed around £5m each for the last four years.
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 give UK addresses, are never asked to pay.
The investigation suggests that just over £21m was recovered by trusts in 2015/16. The departments most commonly used by overseas visitors included maternity and general medicine.
Eight hospital trusts in London are owed more than £1 million and a further 21 across England are owed at least £100,000. These include hospitals in Liverpool, Birmingham, Oxford and Sheffield.
Barts and The London NHS Trust, topped the list in 2015/16 for its unpaid bill, with £4,987,190. The trust managed to recover £6,754,000 in the same year.
London North West Healthcare NHS Trust is owed £1,433,018 from 2015/16. In 2015/16 it invoiced 438 overseas patients for £1,578,289. Almost £40,000 was billed to Americans, who also owed the trust £38,297 in 2015/16.
Some £473,310 was billed to people from India, with Indians also owing the trust £423,151 in 2015/16. The single biggest bill for one patient from India was £28,487. A further £50,455 was billed to people from Nigeria, with Nigerians also owing the trust £45,191 in 2015/16.
People from Pakistan accounted for a bill of £122,887 in 2015/16, with unpaid bills in the same year among people from Pakistan reaching £119,487.
Meanwhile, Kings College Hospital NHS Foundation Trust recovered £1,232,990 in 2015/16 but has written off £2,217,311 for the same year. University College London Hospital is owed £2,393,000.
St George's Hospital in Tooting, South London - which has hit the headlines after pregnant health tourists went there to give birth - managed to recover £159,012 in 2015/16 but had an unpaid bill of £347,019. In total, it is owed £2,550,059 from the last four years.
A spokesman said the figure only represents patients who have been identified as chargeable before, during and after their treatment.
It does not include patients admitted for complex care and prolonged admissions. St George's said it expected its bill to rise "year on year" and it estimates that overseas patients who do not pay actually cost in the region of £4m to £5m per year.
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.
"The money owed to London NHS trusts alone is jaw-dropping."
Dr Mark Porter, chairman of the British Medical Association (BMA), 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."
A spokesman from Barts Health NHS Trust said: "We make every effort to recover costs from eligible patients. We use hand-held card readers to obtain funds from patients at their bedside, and have recently improved our internal processes to more quickly identify paying patients.
"Our four major hospitals include the largest cardiovascular unit in the country, a major trauma centre and one of the largest dialysis units in the UK and care for some of the most seriously unwell patients in London."
A Department of Health spokeswoman said: "The NHS is a national - not an international - health service and we are determined to stamp out abuse of the system to ensure it remains free at the point of need in this country.
"We consulted earlier this year on extending the charging of migrants and visitors using the NHS. We will set out further steps in due course to ensure we deliver on our objective to recover up to £500 million a year by the middle of this Parliament."
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."