Private information stored online by European computer users could be scrutinised by American law enforcement agencies under a wide-ranging new right-to-snoop being pursued by the US Government.
Federal authorities in the US are using the courts to try to force American-owned technology companies to disclose emails and other data held in the "cloud" - the vast network of computers where electronic data is stored for customers.
The claim would require companies such as Microsoft, Apple and Google to open up all their electronic records to agencies such as the CIA, the NSA and the FBI - even if it is stored in Europe rather than on US soil.
A New York court this month ordered Microsoft to hand over to American prosecutors the emails of a European customer stored on its servers in Ireland, as part of a drugs trafficking investigation.
Loretta Preska, the judge, ruled that the technology giant must comply with the US warrant because the company is American, even though it could be breaking Irish and EU law if it were to do so.
Microsoft has been fighting the order, with the latest stage in its appeal due to begin in December.
The company, which is supported by other technology giants, has indicated it will take its battle to the US Supreme Court if it loses.
If the US Government wins the case, data stored by European customers in the cloud would be open to inspection by American investigators.
It would also affect details held about people even if they have never used the internet, as businesses and even government departments use the services of American-owned companies to store information in the cloud.
The insecure nature of cloud storage has already been highlighted by the disclosure of intimate pictures of more than 100 Hollywood actors and other celebrities after they were stolen by hackers from Apple's iCloud service.
John Hemming, a British MP and information technology expert, has now raised fears about the implications for the security of parliamentary data.
The electronic mailboxes of MPs previously held on an in-house parliamentary system were switched in July to Microsoft servers in Ireland and the Netherlands.