The company, which is owned by Chinese giant ByteDance, is challenging a €530 million fine by the Irish regulator last year, when officials found it had allowed Chinese staff to access Europeans’ data — but failed “to verify, guarantee and demonstrate” that the data was properly protected.

The Irish regulator wants TikTok to shut off data flows to China, unless it can prove its user information is safe from Beijing’s invasive surveillance and intelligence laws.

The case is a major test for Europe’s privacy rulebook, the General Data Protection Regulation (GDPR), and how it protects Europeans when their data is transferred to China. It comes as Europe is facing transatlantic pressure, forcing the bloc to revisit trade ties with Beijing, despite long-held security concerns over the Chinese government’s data snooping practices.

Lawyers faced off Tuesday in Dublin’s top courts building, for the start of a grueling 10-day hearing, sparring over how to interpret the limits of Chinese laws and the merits of TikTok’s data practices.

“The consequences of [the Irish regulator’s] decision are immense, even for a very large organization like TikTok,” the firm’s senior counsel Paul Gallagher told the court, estimating the cost of complying with the Irish order to run as high as €5 billion.

If judges side with the Irish regulator, that could ultimately force TikTok to unplug from China entirely to continue serving European users — just months after it split off its U.S. operation into a new app, under the control of a group of investors led by Silicon Valley giant Oracle and investment firms Silver Lake and MGX, to alleviate long-standing American data security concerns.