Uber suffered a setback to its global expansion plans on Thursday when a senior adviser to Europe’s highest court recommended that the ride-hailing company comply with the region’s tough transportation rules, potentially hobbling growth of its service across the Continent.

The nonbinding opinion comes as Uber faces growing pressure worldwide after a string of missteps by its executives angered regulators and raised questions about the leadership of Travis Kalanick, the company’s chief executive.

The European case centers on whether Uber should be considered a transportation service or a digital platform that merely connects independent drivers and potential passengers. By designating Uber a transportation service, Maciej Szpunar, an advocate general at the European Court of Justice in Luxembourg, said on Thursday that the company should comply with European countries’ safety rules, as well as with other legislation that applies to the traditional taxi associations that are Uber’s main rivals across the region.

“Uber cannot be regarded as a mere intermediary between drivers and passengers,” Mr. Szpunar wrote in an opinion that will be reviewed by the European Court of Justice, which is expected to make a final ruling by late summer.