Legal complaints against the UK triggered by Edward Snowden's revelations on global surveillance will today be heard by the European Court of Human Rights (ECHR).
Complaints about the human rights implications of the ability for GCHQ, the UK's signals intelligence agency, to tap the undersea fibre-optic cables which run through the country were first raised in 2013.
They followed the revelation of the existence of a programme called Tempora, based on documents given by Mr Snowden, an ex-US intelligence contractor, which revealed GCHQ had implanted intercepts at key global locations of fibre-optic cables allowing the agency to store internet traffic.
Now, more than four years later, three challenges brought by a number of human rights and privacy organisations have arrived at the Strasbourg court.
The court will establish whether the surveillance is in breach of the European Convention on Human Rights.
The British Government has argued that the bulk interception programme is allowed by law and necessary because of the difficulty of investigating international crime and terrorism.
Scarlet Kim, legal officer at Privacy International, said: "For years, the UK Government has been intercepting the private communications and data of millions of people around the world.
"At the same time, it can access similarly enormous troves of information intercepted by the US Government.
"These practices are unlawful and violate the fundamental rights of individuals across the world, assailing privacy and chilling thought and speech.
"They are incompatible with open and democratic societies."
After the hearing, the court will hold its deliberations in private and will make a ruling at a later date.
The European Court of Human Rights sits under the Council of Europe, which was founded in 1949, rather than the European Union, whose forerunner, the European Economic Community, was founded in 1957.
The court aims to uphold the European Convention of Human Rights, which was principally drafted by Sir David Maxwell-Fyfe, a British MP who had been a prosecutor at the Nuremberg Trials.
Written following the two World Wars and in the shadow communism in eastern Europe and the fascism in Germany, Italy and Spain, the Convention aims to establish protections for civil liberties essential to democracy.
It was through a ruling in 1984 that Britain's first ever surveillance law, the Interception of Communications Act 1985, was written.
A challenge at the court in the case of Malone v The United Kingdom found that the Home Office's phone tapping activities were not regulated enough and in breach of the Convention.
Subsequent legal challenges have taken place domestically in the UK and at both the EU's Court of Justice and the European Court of Human Rights, continually prompting reform of the UK's surveillance laws.Let's