Russia must provide clarity over the whereabouts of jailed opposition figure Vladimir Kara-Murza, UK Foreign Secretary David Cameron has demanded.
Concern has grown since the Russian-British national, convicted of treason after criticising the war in Ukraine, was removed from a prison in Siberia.
Russian prison transfers are shrouded in secrecy and can take weeks.
Lord Cameron said Russia "must urgently" inform Kara-Murza's lawyers of his whereabouts.
Enquiries to the penal colony in Omsk where he was held received responses saying he was no longer there.
"I'm deeply concerned for Mr Kara-Murza - a British national imprisoned in Russia for speaking out against the invasion of Ukraine," Lord Cameron posted on X, the social media platform formerly known as Twitter.
The activist's wife, Evgenia Kara-Murza, said she had learnt on Monday that he had been moved "in an unknown direction" from a punishment cell in the penal colony where he had been held since September 2023.
"There are no grounds for his transfer and that makes it even more frightening as my husband is in the hands of the same people who tried to kill him twice, in 2015 and 2017," she said, apparently referring to two alleged assassination attempts when he nearly died after being poisoned.
"I demand that the Russian government provide us with information about my husband's whereabouts," she said.
The British foreign secretary added: "I stand with his wife."
Mr Kara-Murza was sentenced in April to 25 years for spreading "false" information about the Russian army and being affiliated with an "undesirable organisation".
The former journalist and politician, 42, denied all of the charges.
He had spent years speaking out against Russian President Vladimir Putin, and has criticised the government's crackdown on dissent as well as the war in Ukraine.
He had also played a key role in persuading Western governments to sanction Russian officials for human rights abuses and corruption.
Shortly before he was sentenced, he said in remarks posted online: "I know that the day will come when the darkness engulfing our country will clear."
His sentence was condemned by the British government which summoned Russia's ambassador and said it would look at measures for holding those involved in his detention and "mistreatment" to account.
The US state department described Mr Kara-Murza as "yet another target of the Russian government's escalating campaign of repression".
Mr Kara-Murza, who comes from a well-known Soviet dissident family, received British citizenship when he moved to the UK as a teenager with his mother.
When Russian opposition leader Alexei Navalny, who has been in jail since 2021 on multiple charges widely seen as politically motivated, was transferred within the penal system in December, there was no contact with his supporters outside for nearly three weeks.
