Southampton hospital bosses wrote off more than a quarter of a million pounds last year after treating foreign patients who left without settling their bills.
Figures released in Parliament show Southampton University Hospitals Trust wrote off or abandoned claims for £253,000 in 2011.
The trust, which needs to save £100 million by 2015, is also owed almost £500,000 in unpaid bills going back even further.
Bosses have said foreign patients who were “unwilling or unable” to pay would only receive treatment if it was urgent.
But Chris Skidmore, the MP who compiled the figures, said: “The NHS is a national health service, not an international one.”
The Tory MP is campaigning to change the law so that more can be done to secure payment before treatment starts.
Unless their country has a reciprocal arrangement with the UK foreign nationals are required to pay for any non-emergency NHS treatment they receive.
Winchester and Eastleigh NHS Trust, now part of Hampshire Hospitals Foundation Trust, that runs the Royal Hampshire County Hospital, Winchester, said it had received £218,000 from foreign patients in 2010/11, and had not written off any bad debts.
It is estimated that across the country the NHS is owed £40 million, with £14 million written off by health trusts last year.
Payment is usually raised with foreign patients when they are admitted to hospital, but in some cases, for example if they are in a coma, this is impossible.
Southampton University Hospitals Trust said: “We are one of the largest acute hospital trusts in the country and ensuring those who require emergency treatment have access to it immediately is our highest priority.
“Patients we treat who are not resident in the UK and therefore not entitled to free NHS treatment will be charged for any treatment they receive away from the emergency department.
!If they are unwilling or unable to pay, they receive only the treatment that is immediately necessary.”