Rights groups including Amnesty International are taking the UK government and its use of mass surveillance to the European Court of Human Rights, and will face a determined opponent in the form of GCHQ.
The UK's use of such surveillance was revealed in the Edward Snowden leaks and has caused controversy ever since, but it has been repeatedly defended by the government.
Amnesty, Liberty and Privacy International have already challenged the practices with varying degrees of success, and have now increased their opposition.
"The UK government's surveillance practices have been allowed to continue unabated and on an unprecedented scale, with major consequences for people's privacy and freedom of expression," said Nick Williams, Amnesty's legal counsel.
"No-one is above the law, and the European Court of Human Rights now has a chance to make that clear."
The groups have been frustrated by the results so far, and are not prepared to accept the recent findings of the Investigatory Powers Tribunal (IPT) which decided that the practice is compliant with human rights.
The suspicion is that the IPT investigation and report omitted details to which the European courts might pay more attention.
GCHQ denies this, saying that the IPT decision is fair and right and that it will defend this to the end regardless of the court case.
"We completely reject the assertions made [by] Amnesty International and others, which do not reflect the judgements of the IPT," said a spokesperson.
"The IPT was clear in its December judgement that the legal regime is lawful, and that GCHQ does not seek to carry out mass surveillance. The government will be vigorously defending this case at the European Court of Human Rights."
The privacy groups are not likely to drop this, however, and the fight will continue. The groups argue that the practice is contrary to all they stand for, and should be stopped.
"Mass surveillance is a violation of our fundamental rights. Intercepting millions of communications every day, and secretly receiving millions more from the US National Security Agency by the back door, is neither necessary nor proportionate," added Carly Nyst, legal director at Privacy International.
"While the IPT sided with GCHQ and against the rights of millions of people, Europe's highest human rights court has a strong history of ensuring that intelligence agencies are compliant with human rights law.
"We hope that the court continues this tradition and GCHQ is finally held accountable for its unfettered spying on the world's communications."
The intention to challenge the IPT decision was revealed by Privacy International in December. The joint application was filed at the Strasbourg court last week.
"It is ridiculous that the government has been allowed to rely on the existence of secret policies and procedures discussed with the IPT behind closed doors to demonstrate that it is being legally transparent," added Williams.
"This industrial-scale mass surveillance makes it increasingly difficult for organisations like Amnesty to carry out human rights work.
"It is critical that we are able to seek and receive information of public interest from our confidential sources, free from government intrusion."
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