Uber claims it is a service provider, connecting riders with freelance drivers directly and much more cheaply than traditional cab companies.
It has run into huge opposition from critics and competitors who say this allows it to dodge costly regulation such as licensing requirements for drivers and vehicles.
In an opinion on a case brought by a taxi drivers association in the Spanish city of Barcelona, Advocate General Maciej Szpunar of the European Court of Justice said San Francisco-based Uber should be treated as a traditional taxi company.
"Uber can thus be required to obtain the necessary licences and authorisations under national law," he said.
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The ECJ's advocate generals are senior lawyers whose opinion the EU's top court very often follows when it comes to a final ruling.
Uber has had a rough ride in Spain, where a judged ruled in 2014 that its UberPop service risked breaking the law, leading to the Barcelona submission.
Early last year it decided to operate in Spain only a limited a version of its UberX service which uses licensed, professional drivers instead of amateurs.
Licensed taxis must undergo hundreds of hours of training and they accuse Uber of endangering their jobs by using cheaper drivers who only need a GPS to get around.
Szpunar said he believed that Uber could not be considered to be solely an information service which falls under a different regulatory regime.
Rather it was a composite service, providing both information electronically and then the all important means of transport.
"Uber cannot be regarded as a mere intermediary between drivers and passengers," Szpunar said.
Since the transport component is the main part of its offering, Uber "must be classified as a 'service in the field of transport.
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