Bahrain's opposition leader, Sheikh Ali Salman, has been handed a life sentence after the Court of Appeal found him guilty of spying for Qatar, reports BBC.
The ruling comes just months after the Bahraini High Court of First Tier acquitted Salman of the charge of "colluding" with the rival state.
Bahrain cut ties with Qatar in 2017.
Human rights group Amnesty has described the decision as a "travesty of justice" amid Bahrain's "continued crackdown on dissent".
"This verdict... demonstrates the Bahraini authorities' relentless and unlawful efforts to silence any form of dissent," Amnesty's Middle East and North Africa director Heba Morayef said.
"Sheikh Ali Salman is a prisoner of conscience who is being held solely for peacefully exercising his right to freedom of expression."
Ali Salman, who led the now outlawed Al-Wefaq movement, was accused of plotting with Qatar to stoke anti-government unrest in 2011, along with fellow opposition leaders Hassan Sultan and Ali al-Aswad.
The accusation emerged after Bahrain, Saudi Arabia, the UAE and Egypt cut links with Qatar in June 2017, accusing the emirate of supporting terrorist groups and of being too close to Iran - allegations the emirate has vigorously denied.
At the time, Wefaq said the allegations were an attempt by the Bahraini government to smear it and prolong the imprisonment of its leader, who has been in custody since 2015.
Sunni-ruled Bahrain has been wracked by unrest since security forces crushed pro-democracy protests mainly led by the majority Shia community six years ago.
But the royal family of Al Khalifa managed to suppress the protests with the help of neighbouring countries, most importantly Saudi Arabia.
Ali Salman can still appeal against the decision.