The European Commission is taking legal action against Spain over the refusal of some of its hospitals to recognise the European Health Insurance Card.
The EHIC entitles EU citizens to free healthcare in public hospitals.
But some Spanish hospitals rejected the card and told tourists to reclaim the cost of treatment via their travel insurance, the Commission says.
A BBC correspondent says the Commission is not accusing cash-strapped Spanish hospitals of trying to make money.
The Commission, which checks compliance with EU law, has requested information on the issue from the Spanish government - the first stage of an infringement procedure which could eventually result in a fine.
Under the health card system, an EU citizen's home health service is supposed to cover the cost of emergency treatment abroad. So for a British citizen the UK National Health Service picks up the final bill.
The EHIC is mainly intended for emergency treatment abroad, but also covers patients for pre-existing medical conditions. In cases where doctors think a European visitor's treatment can wait until they get home then the EHIC can be deemed invalid.
The BBC's Matthew Price in Brussels says the EHIC is intended to give foreign visitors the same level of care in emergency cases as locals get.
Our correspondent says a British holidaymaker in Spain told him that when he went to hospital severely dehydrated he was told to give them his travel insurance details.
The Commission says it has had hundreds of such complaints concerning Spain. It says the much higher cost of private treatment is being passed on to the travel insurance companies "or, increasingly, is being billed to the citizens directly".
BBC
