Britain's programme of mass surveillance, revealed by whistleblower Edward Snowden as part of his sensational leaks on USA spying, violated people's right to privacy, Europe's top rights court ruled Thursday.
The court is not a European Union institution and is instead part of the Council of Europe, a 47-member state organisation based in Strasbourg, which Britain is not leaving after Brexit.
In 2013, Edward Snowden revealed that GCHQ - the UK's eavesdropping agency - had been secretly collecting communications sent over the internet on an industrial scale.
They wrote: "In view of the potential chilling effect that any perceived interference with the confidentiality of journalists' communications and, in particular, their sources might have on the freedom of the press, the Court found that the bulk interception regime was also in violation of article 10".
The court said this violated the right to privacy under the European convention - as did the way in which GCHQ obtained communications data from service providers.
"Secondly, the absence of any real safeguards applicable to the selection of related communications data for examination".
In contrast, related communications data is capable "of painting an intimate picture of a person" through mapping social networks, location tracking and insight of who they interacted with. "The government will give careful consideration to the court's findings", said a Home Office spokesperson.
Following the Snowden revelations, the rules were replaced in November 2016 by the Investigatory Powers Act, a new law that effectively puts mass surveillance powers on a statutory footing. Today, the Court has ruled the UK's surveillance illegal, despite arguments that the Regulation of Investigatory Powers Act 2000 (RIPA) and its replacement the Investigatory Powers Act 2016 (IPA) made it legal.
"This includes the introduction of a "double lock" which requires warrants for the use of these powers to be authorised by a Secretary of State and approved by a judge", the government said in a statement. "There must be a public interest which overrides the vital right to journalistic free speech before officials can scrutinise such material".
"Under the guise of counterterrorism, the United Kingdom has adopted the most authoritarian surveillance regime of any Western state, corroding democracy itself and the rights of the British public", Carlo said in a statement.
Commenting on the ECHR's ruling, Silkie Carlo, director of Big Brother Watch, said: "This landmark judgment confirming that the UK's mass spying breached fundamental rights vindicates Mr Snowden's courageous whistleblowing and the tireless work of Big Brother Watch and others in our pursuit for justice".
Founders of the Bureau David and Elaine Potter backed the case.
"I have looked pretty carefully at this over a number of years and there is no doubt in my mind that these powers are very useful - not just in counter-terrorism, but in cyber defence and at a much more everyday level they are useful for the police in fighting online sex crime, county lines and in a missing persons investigation".