Uber should be primarily considered a taxi company and not a technology firm, the European Union's top court has ruled.
The decision delivers a fresh blow the the embattled company, and could change the way it functions across the continent.
The case at the the Luxembourg-based European Court of Justice stems from a complaint by a Barcelona taxi drivers association.
Local taxi drivers in the city wanted to prevent Uber from setting up there, and said Uber drivers should have authorizations and licenses.
The ride-sharing app has said the normal cab regulations did not apply to it, since it was primarily an app which connected drivers with passengers.
But it did not convince the court, which said in a statement that Uber must be classified as "a service in the field of transport." The decision could affect such services around the EU.
Uber said the case would not change how it operates in the UK, where both the company and its drivers already have to obtain operating licences from local authorities.
An Uber spokesman said: "This ruling will not change things in most EU countries where we already operate under transportation law."
It's just the latest in a series of setbacks for the San Francisco-based firm.
It has now been barred from operating in three UK cities, including London, though all three cases are undergoing appeals.
Uber is also facing legal challenges from drivers who say they are in fact workers who deserve basic rights including holiday an sick pay.
Uber has also faced sharp criticism after it emerged that it tried to cover up a major hacking attack that saw millions of users' data stolen.