“Interference with fundamental rights is contrary to the principle of proportionality, in particular because the surveillance carried out by the United States intelligence services is mass, indiscriminate surveillance,” he added.

Many technology companies rely on transferring data between regions to power their operations, particularly for online advertising. But European privacy campaigners have balked at this because American data protection rules do not offer the same protection to individuals that is available in Europe, where privacy is viewed as a fundamental right almost on par with freedom of expression.

By suggesting that companies may not be able to move data about their European customers to the United States, the court adviser’s opinion may force companies like Google and Facebook to rely more heavily on their European operations. They might need to build new data centers in Europe to analyze people’s online activities.

Industry executives have warned that companies may not be able to offer new services to Europeans if such offerings relied on sending online data outside the 28-member bloc.

“We are concerned about the potential disruption to international data flows if the court follows today’s opinion,” John Higgins, director general of DigitalEurope, an industry trade group, said in a statement.

While the advocate general’s opinion does not have to be followed by the court, analysts say that in many cases, the judges have followed the advice.

There have been some exceptions, though.

Last year, the European Court Justice ruled that people with connections to Europe could ask that search engines like Google remove links about them from online search queries. That decision on the so-called right to be forgotten went against the advice of the court’s advocate general.