Uber is a transport firm that requires a licence to operate, a senior member of European Union's top court has said.
The decision is a set-back for Uber, which had argued it only provided technology to help drivers find passengers.
If the ruling is enforced across Europe, it might mean Uber has to operate under the same conditions and safety rules as established taxi firms.
Uber said the decision did little to change the way it was regulated.
Maciej Szpunar, advocate general at the Court of Justice of the European Union, made the decision while considering a case brought by an association of taxi drivers from Barcelona.
They said Uber was competing unfairly in the city by using unlicensed drivers for its service.
Mr Szpunar said that although Uber was "innovative", the way it operated still fell within the realm of transport rather than information services.
Uber was not merely a middleman, he said, but was essential to the way the ride-sharing system worked.
"Uber can thus be required to obtain the necessary licences and authorisations under national law," said Mr Szpunar.
The decision is not binding, but in the past judges ruling on cases before the court have generally followed the advocate's lead.
For Uber, the decision might mean it has to take more care of its drivers and ensure they are properly trained.
The decision could also have implications for other tech companies, such as Airbnb and Deliveroo, that maintain a "hands-off" relationship with people who provide their services.
In response, Uber said it was already regulated as a transport operator in many European countries so the ruling would have little impact.
But it added that if the court backed the ruling it would "undermine the much needed reform of outdated laws which prevent millions of Europeans from accessing a reliable ride at the tap of a button".
Analysis: Theo Leggett, business correspondent
This isn't a final ruling - although judges at the ECJ do follow the reasoning of the Advocate General in most cases.
Uber says if it loses the case, its operations in most countries won't be affected. This is true - Uber has spent the past few years fighting a raft of legal cases, and has generally found a solution acceptable to national and local authorities.
But it still regards the rules in many areas as "outdated and disproportionately restrictive". It maintains that it's a different kind of company to traditional taxi firms, with a new business model, and it says laws should be updated to take that into account.
That idea doesn't go down well with many European taxi driver unions, which regard Uber as an interloper, intent on undercutting their members and determined to use any legal loopholes it can find to do so.
If the court decides that under European law Uber is not simply an intermediary - a platform for putting passengers in touch with drivers - but is in fact a transport company, it will set an important precedent, which national governments will be able to refer to. In law, it will effectively be just another taxi firm.
That, the company thinks, could undermine the "much needed reform of outdated laws" which it is anxious to promote.
- 1 May 2017
- 3 August 2016
- 28 March 2017
- 4 May 2017