The European Court of Human Rights has found the 1999 trial in Turkey of Kurdish rebel Abdullah Ocalan was unfair.
The move raises the possibility of a new trial that could inflame nationalist passions aroused by his capture six years ago.
Ocalan was found guilty of treason by the Turkish court over the separatist revolt in the southeast in which at least 30,000 people were killed in the 1980s and 1990s. Fears of a revival of the war have haunted the country since his Kurdistan Workers Party ended a unilateral ceasefire last June.
The ruling by the court in the French city of Strasbourg still needs formal approval but may cause immediate problems for Turkey's centre-right government, which is
already grappling with nationalist unease over Europe that could itself delay entry talks with the EU.
"The applicant was not tried by an independent and impartial tribunal," the panel of judges said in a statement read out at the court after reaching their verdict by a vote of 11 to 6.
The judges said Ocalan, serving a life sentence as the sole inmate of a Turkish island prison, had not had proper access to legal counsel or the facilities needed to prepare his defence.
Dozens of Kurds cheered and chanted outside the court under a banner declaring "Free Ocalan - Peace in Kurdistan".
"The court has universally declared that there was no fair trial and in those circumstances it is pointing the way towards a retrial," Mark Muller, a lawyer for Ocalan, told reporters.