LONDON — Three years after former NSA contractor Edward Snowden fled to Russia, the fallout from the documents he leaked can still be felt quite strongly.

In Europe, the biggest repercussions might still be ahead.

A lawsuit filed by 10 organizations challenging U.K. mass surveillance has now reached the European Court of Human Rights in a development that constitutes the most significant legal challenge based on the Snowden revelations outside the United States, so far. The court's judgments are legally binding. Although Britain has decided to leave the European Union, the country would have to separately withdraw from the European Convention on Human Rights or resign as a member of the Council of Europe to ignore any ruling.

Human rights advocates hope the court will rule in their favor, in part because such legal challenges have been successful in the past. The European Court of Justice, another institution, struck down transatlantic plans to allow private companies to share personal data on individuals last year. In an explanation, the court cited the Snowden revelations.

A decision in favor of the 10 NGOs could impact American intelligence gathering that relies on access to British data, as well as its technical operation centers, which are believed to be located in Germany.

In their court challenge, the NGOs argue that "this case is of critical importance to the privacy of modern forms of communication used by billions of people around the world." According to them, domestic courts have "failed to challenge the U.K. Government on whether it can ever be lawful to surveil en masse individuals, groups and organizations not suspected of any crime."

The case also challenges British access to U.S. intelligence under the so-called five eyes agreement that allows extensive information sharing between Australian, Canadian, New Zealand, British and American officials. Over the last three years, U.K. and American intelligence agencies have rebutted claims that their intelligence gathering has violated laws.

According to Caroline Wilson Palow, the general counsel at human rights organization Privacy International, mass surveillance is incompatible with the rights to privacy and freedom of expression that are among the leading principles defined by the European Convention on Human Rights, which Britain has signed.

Following the Snowden revelations, the British government vowed to overhaul its intelligence gathering. The so-called  Investigatory Powers Bill is still being debated, but critics have already described it as an attempt to legalize procedures that have long been secretly in place.

Speaking to The Washington Post's Karla Adam last year, Wilson Palow said the British proposals highlighted a “big contrast” with trends in the United States.

"The U.S. is trying to rein in bulk collection, or what we might call mass surveillance," Palow said. "The U.K. is going in the opposite direction, enshrining the ability for mass surveillance in law."

The debate about the Investigatory Powers Bill has been focused on preventing crimes, but the underlying rhetoric has worried human rights advocates.

In November, Theresa May, who was then home secretary and is now prime minister, defended the controversial bill, saying it was the "modern equivalent of an itemized phone bill."

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