Skype is being investigated by Luxembourg's data protection commissioner over worries about its secret involvement with the NSA.
The Microsoft-owned video chat company could potentially face criminal and administrative sanctions, including a ban on passing users' communications covertly to the US signals intelligence agency.
Skype itself is headquartered in the European country, and could also be fined if an investigation concludes that the data sharing is found in violation of the country's data-protection laws.
Both Microsoft and Skype have declined to comment on the issue.
Luxembourg has attracted several large corporations, including Amazon and Netflix, due to its tax structure.
Surveillance of communications in Luxembourg can only occur with judicial approval or by authorisation of a tribunal selected by the prime minister.
However, it is unclear whether Skype's transfer of communications to the NSA have been sanctioned by Luxembourg through a secret legal assistance or data transfer agreement that would not be known to the data protection commissioner at the start of their inquiry.
Microsoft's acquisition of Skype tripled some types of data flow to NSA, according to top-secret documents seen by the Guardian. In July last year, nine months after Microsoft bought the internet phone company, the NSA boasted that a new capability had tripled the amount of Skype video calls being collected through Prism.
Last month Microsoft revealed their figures concerning government requests for data in the first six months of 2013, with 66,539 of the company's user accounts requested for scrutiny.