The Maldives government has backtracked on a promise to allow jailed ex-leader Mohamed Nasheed to travel to the UK for surgery, his lawyer and party say.
Nasheed was due to leave for the UK for back surgery on Sunday after the government agreed to his request.
But it has now insisted that he nominate a family member to stay in the capital, Male, to guarantee his return, said lawyer Hassan Latheef.
The government said this was standard for convicts travelling abroad.
Nasheed was given a 13-year sentence under anti-terror laws last year - following a trial which his lawyers said was biased and intended to end his political career.
The government had only agreed to the travel on Saturday following international pressure, saying he must return to the Maldives to serve the remainder of the sentence.
The former leader has since cancelled his travel plans.
'Effectively a hostage'
Mr Latheef told Reuters: "The government reneged on the agreed deal at the last minute, demanding a close family member of Nasheed remain in Male, effectively as a hostage, until he returns from the UK."
"If Nasheed does anything that breach[es] the terms of the government, the family member could then be criminally prosecuted. This kind of blackmail is illegal, unheard of in international affairs, and totally outrageous."
Hamid Abdul Ghafoor, a spokesman for Nasheed's Maldivian Democratic Party, told AFP that the family member could face prosecution if he did not return after 30 days.
"It is an ethical issue. That is why president Nasheed has not agreed to the demand of a guarantor," he said.
Local media quoted justice department officials as saying Nasheed could travel as soon as he signed the relevant documents nominating a family member.
A former human rights campaigner, Nasheed became the nation's first democratically elected leader in 2008, ending three decades of rule by former strongman Maumoon Abdul Gayoom.
In 2012, he was detained after being accused of ordering the arrest of a judge.
He resigned months later amid an army mutiny and public protests over the judge's fate.
Nasheed alleged that he had been removed by a coup, but this was denied by his vice-president, who replaced him.
The current President Abdulla Yameen was elected in controversial polls in 2013 and is the half-brother of Mr Gayoom.