Kuwait's supreme court on Monday upheld a two-year jail sentence against prominent opposition leader Mussallam al-Barrak for insulting the emir, his group said.
"The supreme court upheld the two-year jail term against Barrak," the Popular Action Movement (PAM) said on its Twitter account, adding that the former lawmaker, who had been out on bail, will now have to serve the full sentence.
The 58-year-old is the most senior and the most popular opposition member to be jailed in an unprecedented government crackdown on dissent.
He was convicted by the appeals court in February of undermining the authority of Emir Sheikh Sabah al-Ahmad Al-Sabah, and served 50 days of the jail sentence before being bailed.
The supreme court verdict is final and cannot be challenged.
It may also jeopardise his political career because it could bar him from standing for election to parliament.
Barrak blasted the ruling as politically motivated.
"This is undoubtedly a political ruling of the first degree," he told supporters who gathered at his residence after the verdict.
Barrak, who was a lawmaker between 1996 and 2012, complained that the court did not provide the required legal procedures for a fair trial.
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However, he insisted he will continue the struggle.
"I chose to become a prisoner of opinion," Barrak said, and called on his supporters to step up peaceful political struggle and to avoid violence.
The charges against Barrak stem from a speech he gave to tens of thousands of demonstrators in October 2012, protesting against changes to the electoral law.
He said the change would allow the government to manipulate the outcome of elections.
Barrak was a member of parliament at the time, but his nationalist party boycotted polls in December 2012 and July 2013 held under the new electoral law.
Online and opposition activists strongly criticised the verdict.
Islamist former lawmaker Waleed al-Tabtabai lamented on Twitter that the "era of jailing politicians has started".
The Islamist Ummah Party said in a statement that Barrak's imprisonment "proves that Kuwait needs a new constitution".
The appeals court sentenced Barrak after quashing a five-year jail term handed down by a lower court.
Kuwaiti courts have sent dozens of opposition and online activists to jail on charges of insulting the ruler.