A 75-year-old woman has won a landmark European Court ruling that patients must be reimbursed for treatment abroad if they face "undue delay" for surgery on the NHS at home.

Yvonne Watts, who paid almost £4,000 for a hip replacement operation in France, had been told she would have to wait a year for an operation on the NHS - a period later reduced to four months. But when she sent the bill for her treatment in France to the Bedford Primary Care Trust, it refused to refund the cost.

The European judges criticised NHS rules on treatment abroad for taking too little account of individual cases. They did not award Mrs Watts a refund, saying it was up to the British courts to decide if she had faced an "undue delay". The case will go back to the Court of Appeal in England to decide if she should be refunded her medical costs.

Lawyers said that the ruling gave the green light for "health tourism" by patients facing long waits who were prepared to go abroad and later reclaim the cost.

NHS managers said its effect would be limited as doctors were less likely to say that patients needed more urgent treatment than the NHS could provide. Mrs Watts pledged to give any refund she won to a medical charity.

The ruling by the European Court of Justice in Luxembourg said that the British Government had wrongly interpreted patient's rights to get treatment in other countries.

Mrs Watts was told in 2002 that she had to wait a year for treatment but, after complaining of severe pain, she was reassessed by a consultant in January 2003 who said that she needed the operation within three or four months. She decided that was still too long and travelled to France for the operation and later billed the NHS for the cost.

Bedford Primary Care Trust argued that, as treatment was offered within the Government's NHS targets, it was not an undue delay. But the European judges ruled that, just because waiting list targets had been met, it did not necessarily mean a patient had not faced an undue delay.

The judges said primary care trusts must ensure a patient's waiting time "does not exceed the period which is acceptable in the light of an objective medical assessment" of clinical need.

The effect of the ruling is to leave it to doctors to decide what constitutes an undue delay.

Gill Morgan, chief executive of the NHS, said the fall in NHS waiting lists meant more patients were being treated more quickly than ever. "This is a landmark judgment, but its effect on the NHS won't be big," Dr Morgan added. "If NHS waiting lists were 18 months there would be an issue but as they decline towards 18 weeks [the Government target by 2008] it becomes less of an issue."