The European Court of Justice has ruled that bulk phone and email data can only be retained if it relates to serious crimes and should not be collected otherwise.
The findings come after ex-backbench Tory David Davis, and Tom Watson, Labour's deputy leader brought a claim to the court, saying it was unjust for the GCHQ, or any other organisation claiming to need access for legal issues to collect such sensitive information in bulk.
However, Davis withdrew his complaint after he was appointed to the cabinet, evidently not wanting to upset his peers.
"Solely the fight against serious crime is an objective in the general interest that is capable of justifying a general obligation to retain data, whereas combating ordinary offences and the smooth conduct of proceedings other than criminal proceedings are not," The ECJ's advocate general Henrik Saugmandsgaard said.
This opinion brings the European Union's stance closer to the regulations set out in the UK's Investigatory Powers Bill, which could come into question when the UK exits the EU.
"This legal opinion shows the prime minister was wrong to pass legislation when she was home secretary that allows the state to access huge amounts of personal data without evidence of criminality or wrongdoing," Tom Watson, Labour's deputy leader said.
"Labour has already secured important concessions, but I hope the government she leads will now revisit it. The opinion makes it clear that information including browsing history and phone data should not be made available to the security services and other state bodies without independent authorisation. The security services have an important job to do, but judicial oversight is vital if we are to maintain the right balance between civil liberties and state power."
The European Court of Justice will come to a final decision about a course of action in the next few months.
