The government must take "urgent action" to recover more money for treating foreign patients, a group of MPs has said.
A Public Accounts Committee report says the system for recouping costs from overseas patients is "chaotic".
Chairwoman Meg Hillier attacked the government's "failure to get a grip" as "simply unacceptable".
The Department of Health said it would be announcing "further steps very shortly to recover up to £500m a year".
Ms Hillier, MP for Hackney South and Shoreditch, said the NHS was missing out on "vital funds".
"The public rightly expects the government to enforce the rules, and more can and should be done to recover money," she said.
The report calls on the Department of Health to publish an action plan by June, "setting out specific actions, milestones and performance measures for increasing the amount recovered from overseas visitors".
NHS hospital care in England is free for UK residents, but those visiting from overseas are charged for non-emergency treatment.
In October, it was revealed the government was expected to fall short of its target of recovering £500m a year from overseas visitors and the Department of Health "refined" its target for 2017-18 to £346m.
Responding to the PAC report, a Department of Health official said: "This government was the first to put measures in place to make sure the NHS recoups money from people who are not eligible for free care.
"Some hospitals are already doing great work, and the amount of income identified has more than trebled in three years, to £289m.
"However, there is more to be done to make sure that if people are not eligible for free care, they pay for it.
"We will be announcing further steps very shortly to recover up to £500m a year by the middle of this Parliament."
Entitled to healthcare?
Hospital trusts in England are legally obliged to check whether patients are eligible for free non-emergency NHS treatment and to recover any costs.
The report identifies the biggest challenge to recovering costs as the lack of a single easy way to prove whether patients are entitled to healthcare.
The committee notes that while some trusts are now requiring patients to prove their identity by showing passports and utility bills, these documents do not demonstrate entitlement to free NHS care.
Some patients, such as refugees and those applying for asylum, are exempt from charges.
The report says the Department of Health should build on existing systems, such as the NHS number and electronic patient record.
There are currently large variations between trusts in the amounts they charge and the debts they recover from overseas patients.
The committee says that trusts' performances should be shared and there should be intervention if a trust is falling short.
And while GP appointments are free to everyone, the report says GPs could do more to help and the government should give clear guidelines on what is expected of them.
Dr Mark Porter council chairman for the British Medical Association, said: "It is important that those accessing NHS service are eligible to do so and that we recover the costs for treating overseas visitors.
"The systems to do this need to be practical, economic and efficient and must not jeopardise access to healthcare for those who need it.
"Any charging systems should not prevent sick and vulnerable patients receiving necessary care, otherwise there may be serious consequences for their health and that of the public in general."