The UK government is facing a legal challenge over its use of the US' Prism programme in the European court of human rights.
Civil rights groups have filed papers arguing that the use of Prism, an internet surveillance scheme which whistleblower Edward Snowden revealed had been tapping private online communications since 2007, breaches the right to privacy found in the European Convention on Human Rights.
It is thought to be the first time a complaint has been made to an international court about either Prism or another previously secret online surveillance operation, Tempora.
Lawyers will claim the regulatory regime underpinning the authorisation of access to 'meta-data' - information relating to the details of an email, message or phone call, rather than the actual content - is dangerously outdated.
They will suggest Britain's intelligence eavesdroppers GCHQ, which used the data but
"The laws governing how internet data is accessed were written when barely anyone had broadband access and were intended to cover old fashioned copper telephone lines," Nick Pickles, director of Big Brother Watch, said.
"Parliament did not envisage or intend those laws to permit scooping up details of every communication we send, including content, so it's absolutely right that GCHQ is held accountable in the courts for its actions."
MPs on the intelligence committee concluded before the summer recess GCHQ's use of Prism did not breach UK law.
A minister signed the warrant for interception each time GCHQ sought information from the US, the committee's report found.
A separate probe will seek to investigate the content of the interceptions requested by GCHQ from American sources.
But Big Brother Watch, the Open Rights Group and English PEN want a judge to rule the spy agency has acted illegally by collecting vast amounts of data.
"Following the revelations about the extent to which GCHQ and the NSA have been harvesting our personal data, no citizen in the UK can be confident that their communications are private," English PEN's director Jo Glanville said.
"If this legal challenge is successful then I'm hopeful that we will secure effective protection of our rights."
A legal challenge in the English courts is barred and a complaint could only be made to the investigatory powers tribunal (IPT), which sits in private.
Lawyers hope the case can be heard on the continent because the European court has previously ruled the IPT does not provide an effective remedy.