The Maldives Supreme Court on Monday upheld a 13-year prison sentence passed last year on the country's first democratically-elected president Mohamed Nasheed, who is living in exile in Britain.

Nasheed was sentenced in March 2015 after he was convicted on a terror-related charge which the United Nations said was politically motivated.

Facing intense international criticism, the government of the Maldives took the unusual step of appealing the conviction based on Nasheed's claim that he did not have adequate time to prepare for the trial.

But Judge Abdulla Saeed ruled on Monday that Nasheed, now leader of the opposition, had been given "sufficient time to present his defence".

There was no immediate comment from Nasheed. He recently won political asylum in Britain, where he went for urgent medical treatment earlier this year.

Nasheed, a climate change activist who was also imprisoned during the three-decade rule of former strongman Maumoon Abdul Gayoom, was elected president in 2008.

He rose to international prominence when he hosted a cabinet meeting underwater to draw attention to the threat global warming posed to the atoll nation's existence.

But he was toppled in February 2012 after a mutiny by police and troops which followed weeks of protests over the arrest of a top judge for alleged corruption as well as for politically motivated rulings.

Nasheed's decision to order the arrest was the centrepiece of the terrorism case against him.

President Abdulla Yameen, a half-brother of former strongman Gayoom, has been criticised over the jailing of Nasheed, whom he beat in a run-off presidential election in late 2013. He has ensured that all his opponents are either in jail or in exile.

US Secretary of State John Kerry warned in May last year that democracy in the Maldives was under threat, saying Nasheed had been "imprisoned without due process".

Since Nasheed's jailing, several others -- including Yameen's former aide and deputy Ahmed Adeeb -- have been given long jail terms.