The European Court of Human Rights on Tuesday condemned Turkey for convicting a teacher in the wake of the 2016 attempted coup on the grounds he had downloaded an encrypted messaging app linked with the alleged plotters.
The ruling that the rights of Yuksel Yalcinkaya had been violated could create a significant precedent, with thousands of similar cases pending before the Strasbourg-based court.
Turkey has always blamed a group led by the US-based preacher Fethullah Gulen for being behind the failed attempt to oust President Recep Tayyip Erdogan, alleging a messaging app called ByLock was used to coordinate the plot.
Thousands of people were arrested and convicted in the wake of the coup on charges of links to the group of Gulen - who has denied any involvement - in a crackdown that caused anxiety among Turkey's allies.
The ECHR, in a damning ruling, said "anyone who had used ByLock could, in principle, be convicted on that basis alone of membership of an armed terrorist organisation".
Meanwhile, there were "procedural shortcomings" in the trial of Yalcinkaya who was arrested in 2016 and convicted in 2017, breaching his right to a fair trial, it added.
Turkey considers Gulen's organisation to be a terror group and has repeatedly called for the preacher - a former ally of Erdogan - to be extradited from the United States.
The ECHR said there were currently 8,500 applications of a similar nature and "given that the authorities had identified around 100,000 ByLock users, many more might potentially be lodged".
It said the failure of the Turkish judiciary concerning evidence of the use of ByLock was "systemic" and Turkey had to take action to tackle these "systemic problems."
This was a final verdict issued by the Grand Chamber of the ECHR, whose rulings are binding for all 46 members of the Council of Europe.
However Ankara has in recent years flouted a succession of rulings by the court, notably concerning anti-Erdogan figures, the Kurdish politician Selahattin Demirtas and philanthropist Osman Kavala - who remain in jail despite ECHR rulings that their rights were violated.