The European Court of Justice has said the NHS could not refuse to refund costs if patients wait longer than clinicians advise, even if targets were met.
The debate centres on where and when a patient has their operation
The court's ruling was triggered by the case of Yvonne Watts, 75, of Bedford, who paid £3,900 for a hip operation in France.
RICHARD STEIN, YVONNE WATTS' SOLICITOR
"The European Court of Justice has said, without a doubt, that people from the UK will now be able to go abroad for treatment if they face undue delay.
"The important thing is that the ECJ has said the decision about whether a patient faces undue delay is a clinical one, rather than an administrative one.
"Traditionally in the NHS, you tend to go to the end of the queue unless you've got a particularly urgent case.
"Now, when deciding what a reasonable waiting period is, the clinician has to decide with reference to you the patient, looking at the pain you are in and how debilitated you are."
DR ED BORMAN, CHAIRMAN OF THE BRITISH MEDICAL ASSOCIATION INTERNATIONAL COMMITTEE
"The BMA welcomes the court's view that government targets alone should not determine how long people have to wait for treatment.
"When patients seek treatment overseas, doctors should be free to make a decision based on their clinical need.
"What we need now is clear guidance from the government about the way patient mobility will work in practice.
"GPs need to know how a patient should be referred to another member state for treatment, and the NHS needs to set out regulations for the reimbursement of cross-border care.
"It is important that the European Commission is now ready to address the wider issues of doctors' liability and patient safety.
"The ruling also highlights the need for continued investment in the NHS so we can treat patients as close to home as possible."
DR SCOTT GREER, UNIVERSITY COLLEGE LONDON
Dr Greer, an expert on the influence of Europe on UK health policy, said the court ruling was both "inevitable" and extremely serious because it now means we really have a European health system.
"Standards of care are not going to be uniform.
"But what is going to be uniform is a regulatory framework which is extremely patchy, because it's based on a few court cases, instead of legislation or a coherent framework."
He added: "Not many British patients will use this ruling. Where we do have patient mobility in continental Europe, the numbers are very small."
"But every part of the NHS will have to ensure it is compliant with regulations which are still very vague.
"The previous court decisions were about countries in which you had to pay part of your health costs.
"In the UK, the NHS is free and universal at the point of treatment which means it rations by waiting.
"And there's got to be some financial way to ration if there's to be any control over the NHS budget.
"Watts has made traditional rationing by waiting much more difficult."
KATHERINE MURPHY, PATIENTS ASSOCIATION
"We must ask ourselves why are people opting to go abroad? Why don't they want to stay with the NHS in this country?
"What this case highlights is that we should be looking at improving our NHS here and making it a service that patients are proud of and want to use.
"I don't think we should always be focused on meeting targets.
"Mrs Watts was obviously in a lot of pain and her quality of life was obviously affected.
"In this case, her GP should have researched and seen if there was anywhere else in the UK that would have performed this without her having to wait."
Ms Murphy said the issue of patients going abroad for treatment raised the issue of cost and the possibility of feeling isolated from friends and family and who provides aftercare.
GEOFF MARTIN, HEALTH EMERGENCY
"Today's court ruling comes at a time when NHS services in the UK are under the heaviest financial pressure for more than a decade.
"We have no doubt that the current round of cutbacks will increase waiting times and that will see a wave of patients heading to Europe for immediate treatment following today's ruling.
"More money will bleed away from the NHS as the cash follows the patient overseas and that will store up even worse financial problems for the future."
DR GILL MORGAN, CHIEF EXECUTIVE OF THE NHS CONFEDERATION
"This is a sensible judgement. It is absolutely right that people are treated on the basis of clinical need.
"As the NHS moves towards treating every patient within 18 weeks, problems like this should cease to be an issue.
"If patients do need to wait, then trusts will ensure that those patients are kept under review in case their situation should deteriorate.
"The service is striving to become more patient-led.
She added: "It seems unfair that patients who can afford to pay for treatment abroad and then claim back costs from their PCT take advantage of this situation.
"This could potentially have a negative impact on those patients who cannot afford to do this when the services in this country aren't as accessible.
"It must be wrong and completely inequitable to have a system where it is more affluent patients who receive treatment quicker as opposed to those with the highest clinical need."
KAREN JENNINGS, UNISON
"This is the third time the European courts have said that people should be reimbursed for medical treatment if they have had to seek that treatment in a neighbouring EU country.
"This is no way to develop health policy. Health care priorities will end up skewed if the NHS has to reimburse those who are prepared to travel abroad for what may be less urgent operations.
"The EU needs to be more strategic about health policy because you cannot rely on the courts to develop it in a way that is in the best interests of all.
"Surely the answer has to be to build up capacity in our own NHS, so that people don't have to wait in pain for too long and be forced to go abroad to seek treatment."
DR MICHAEL DIXON, NHS ALLIANCE
"What this seems to do is to take the decision-making from on these difficult issues away from local NHS teams and they can be made elsewhere."
He said most patients would want to be treated sooner than the date NHS could give them.
"Most of my patients waiting for operations and other treatments would say they are having to wait unduly.
"In an ideal world, patients would be treated immediately and 100% of operations would be successful.
"But this is the real world. And if someone is treated early, someone else will have to be treated late."
TERRENCE O'DWYER, STOCKHOLM NETWORK (EUROPEAN THINK-TANK)
"The ECJ ruling is a victory for patients.
"It is an encouraging sign that Europeans are becoming consumers with real legal rights of choice.
"The option of publicly funded treatment abroad will encourage efficiency and greater quality of care for patients."