The NHS is falling hundreds of millions of pounds short in its its ambition to claw back money lost through health tourism, a report has revealed.
Analysis from the National Audit Office (NAO) showed that while hospital trusts in England were getting back more money from foreign visitors who were not entitled to free treatment, the health service was on course to miss its targets
Estimates suggest that of the £500 million the Government aspires to recover annually by 2017-18, only around £295 million is likely to be brought in from charges of £346 million.
This, the report said, is "significantly less" than the NHS is capable of recovering from the cost of treating patients from overseas.
It showed attempts to reclaim costs to the health service have been hindered by the failure to identify and charge foreigners from the European Economic Area (EEA) who receive treatment and "particular challenges" in pursuing patients outside of the continent.
The "ambitious" target for reducing the cost of treating overseas patients was announced by the Department of Health in 2014 in a drive to cut the deficit facing trusts while countering claims the NHS was being "overly generous".
Of the money charged to overseas visitors in the last year, half the total has come from 10 acute and specialists trusts in London, the NAO said.
Eight trusts, which were not named, had not charged anything at all to patients outside of Europe.
Similarly, while some trusts were found to have a team working specifically on recouping these losses, four had no designated staff for it.
But, the NAO said, reasons for the regional disparity could range from the type of trust to the location of it.
The amount collected has risen from £73 million in 2012-13 to £289 million in 2015-16.
However, the NAO said the increase was mainly due to a new surcharge on temporary migrants from outside the EEA which brought in £168 million last year.
It estimated that only about half of the money charged by trusts is recovered from patients who are slapped with a bill directly, most of whom are from outside of Europe.
Meanwhile the vast majority of the costs are reclaimed from patients who come from inside Europe, due to the countries in question being billed directly - but many are not identified as being from abroad by professionals.
This means the true price could be much higher - as calculations suggest several hundred million more is never recovered due to patients not being identified as being from overseas.
Estimates from 2012-2013 suggested £367 million should have been charged to foreign patients for treatment - but only £49 million was asked for.
A survey from earlier this year showed 42% of hospital doctors and 55% of hospital nurses did not know some people were chargeable, the report said.
A shortfall from the Government's target could also be caused by the costs of implementing the recovery programme, the NAO said.
Financial incentives are believed to have also stumped up funds, as previously some trusts are believed to have pursued overseas visitors because it was more expensive for them to do so.
The head of the NAO, Amyas Morse, said: "Hospital trusts remain some way from complying in full with the requirement to charge and recover the cost of treating overseas visitors.
"In the past two years, the amounts charged and amounts actually recovered have increased. Much of this increase is the result of changes to the charging rules.
"If current trends continue and the charging rules remain the same, the department will not achieve its ambition of recovering up to £500 million of overseas visitor income a year by 2017-18 and faces a potential shortfall in the region of £150 million."
A Department of Health spokeswoman said: "This Government was the first to put in place measures to ensure the NHS isn't abused, and as this report finds, we are making very good progress - the amount of income recovered has already more than trebled in three years to £289 million.
"We consulted earlier this year on extending the charging of migrants and visitors using the NHS to other areas of healthcare.
"We will set out further steps in due course to ensure we deliver on our objective to recover £500 million a year by the middle of this Parliament."