A ruling on UK patients' rights to get treatment in the European Union paid for by the NHS is due from the European Court of Justice.
Yvonne Watts paid £3,800 for a hip operation in France
It follows the case of a grandmother who paid £3,800 for a hip replacement in France as she felt she could wait no longer for an NHS op.
Arthritis sufferer Yvonne Watts, 75, of Queen's Park, Bedford, argued she had been forced to wait unduly.
Her local primary care trust said it was not obliged to reimburse the costs.
The European Court of Justice in Luxembourg was asked to rule on the case after the English Court of Appeal sought its guidance.
The ECJ is expected to say the UK government wrongly interpreted patients' rights to access services in other EU countries.
Mrs Watts, who is severely disabled, was told in September 2002 that she needed a double hip replacement and that she would have to wait around a year for the treatment.
But by the end of January 2003, her condition had worsened and a consultant said she should be operated on within three or four months.
The primary care trust again then refused authorisation for treatment abroad saying she could receive it on the NHS within the "appropriate time". This was upheld in the High Court.
But she decided to go ahead with an operation in the French town of Abbeville in March 2003 nonetheless.
There then followed a High Court battle between Mrs Watts and Bedford Primary Care Trust over the cost of the operation.
Both Mrs Watts and the Department of Health then appealed to the Court of Appeal, which referred the matter to the European Court of Justice.
The Department of Health has argued that if all NHS patients were guaranteed reimbursement when they opted for treatment abroad it could undermine its system of prioritising cases on the basis of clinical need - the waiting lists.
A senior legal adviser to the ECJ, Advocate General Geelhoed, said in December that authorisation for such treatment may not be refused if the treatment cannot be granted without "undue delay".
He argued the position taken by the UK government breached the European treaty on the free movement of people, services, labour and capital.
Medical services were not exempt from the scope of the treaty, the Advocate General said.
But the absence of a clearly defined procedure within the NHS for considering applications for treatment abroad restricts patients' freedom to receive services and is contrary to the EU treaty, he concluded.
He also argued the NHS system for using waiting lists and targets for administering medical priorities could not justify a refusal for authorisation to receive treatment abroad.
Instead the issue of whether a patient was experiencing "undue delay" should be interpreted more broadly.
This should be done by looking at their individual circumstances and taking into account whether the condition of the patient was such that any wait would be unacceptable.
"Targets for providing treatment do not, in view of their abstract character, comply with this criterion," he said.
The Department of Health said it would be reviewing its guidelines in response to the ruling and issuing new guidance in June.
A spokeswoman said: "This ruling will only affect only a handful of people."
She also predicted the European court would take into consideration issues about fairness and access to treatment.
Legal experts say the ruling could have implications across Europe.
Mike Dixon, chairman of the NHS Alliance, which represents primary care trusts said the likely ruling was highly worrying as it could mean patients with the money to pay upfront could jump the queue.
However, chief executive of the Bedford Primary Care Trust Margaret Stockman, said it remained to be seen if the ruling prompted a "free for all" or whether patients in exceptional circumstances would be able to go to Europe for treatment.
The British Medical Association said the ruling could have far-reaching implications for the UK.
Dr Edwin Borman, chairman of the BMA's International Committee, said: "The NHS has finite resources and could suffer financially if it is obliged to fund treatment overseas."