Bahrain in 2017 cut all ties with Qatar as part of a Saudi-led boycott in response to what Riyadh and its allies say are Doha's policies on Iran and Islamist groups
Bahrain's Supreme Court, whose verdicts are final, on Monday upheld a life sentence for Shia opposition leader Sheikh Ali Salman for spying for Gulf rival Qatar, the public prosecutor said.
Salman, who headed the Shia Al-Wefaq group, was convicted by an appeals court in November in a ruling that rights groups called a travesty.
The supreme court confirmed the verdict against Salman and two of his aides for "spying for a foreign state in order to... overthrow the government," according to a statement released by public prosecutor Osama al-Awfi.
Bahrain in 2017 cut all ties with Qatar as part of a Saudi-led boycott in response to what Riyadh and its allies say are Doha's policies on Iran and Islamist groups.
A tiny Gulf archipelago, allied with the US and located between regional rivals Saudi Arabia and Iran, Bahrain has been hit by waves of unrest since 2011, when security forces crushed Shia-led protests demanding a constitutional monarchy and an elected prime minister.
Ruled for more than two centuries by the Sunni Al-Khalifa dynasty, Bahrain has a majority Shia Muslim population according to unofficial estimates contested by the government.
Bahraini authorities accuse Tehran of inciting anti-government rallies and have said jailed protesters have been trained in and armed by Iran, which denies the allegations.
Aides sentenced in absentia
Salman's aides Ali al-Aswad and Hassan Sultan, who had been sentenced to life in absentia, also lost their appeals Monday. Both men are former members of parliament and reside outside of Bahrain.
Qatar has repeatedly denied accusations of conspiring against Bahrain with Salman.
The Gulf diplomatic row is now in its second year. Doha has denied accusations it supports Iran and radical Islamist groups. Bahrain, Saudi Arabia and the United Arab Emirates have banned their citizens from travel to Qatar.
Salman's Al-Wefaq was dissolved by court order in 2016. The cleric is currently serving a four-year sentence in a separate case on charges of "inciting hatred" in the kingdom.
The leftist opposition National Democratic Action Society, or Al-Waad, was banned the following year over allegations of links to terrorists.
Both opposition groups are denied representation in parliament.
Human rights groups have frequently said cases against activists in Bahrain - men and women, religious and secular - fail to meet the basic standards of fair trials.
Amnesty and Human Rights Watch have categorised Salman and other jailed opposition leaders as prisoners of conscience, slamming the initial sentencing of the sheikh and his aides as political reprisal.
Amnesty has called the verdict against Salman "a travesty of justice."
In June, Bahrain amended its law on political rights, prohibiting leaders and members of dissolved political associations from running in legislative elections.
The king last year signed off on a decree granting military courts the right to try civilians accused of "terrorism", a vague legal term.
Bahrain, a vital link for Western militaries, is home to the US Navy's Fifth Fleet as well as a permanent British base.
US President Donald Trump has eased restrictions on arms sales to Bahrain since taking office.
Washington had urged Bahraini authorities not to overturn Salman's acquittal by a high criminal court in June of spying for Qatar, a verdict the public prosecution appealed.