Britain on Wednesday called on Russia to release jailed Kremlin critic Vladimir Kara-Murza for "urgent medical treatment".
Kara-Murza, one of the most prominent opponents of President Vladimir Putin, last year was sentenced to 25 years in prison -- the longest known punishment for a Russian dissident in recent times.
The dual British-Russian national was charged with "treason" after using a speech in the United States to say Russia had committed "war crimes" against Ukraine.
The 42-year-old suffers from serious health problems, which his wife Evgenia and lawyers say are due to two poisoning attempts orchestrated by Russia's FSB security service in 2015 and 2017.
"Imprisoned on politically motivated charges by the Russian authorities, Mr Kara-Murza is being persecuted for opposing Russia's invasion of Ukraine," Nusrat Ghani, Britain's minister of state for industry and economic security, said in a statement.
"The prison conditions are threatening his life. His health is declining," she said after speaking with Evgenia Kara-Murza.
"Russia must release him immediately on humanitarian grounds for urgent medical treatment."
Fears have grown over Kara-Murza's fate following the death of Russian opposition politician Alexei Navalny in an Arctic prison colony in February.
Evgenia Kara-Murza told reporters in Geneva in March that the poisoning attempts had left her husband with a serious condition called polyneuropathy, which can lead to paralysis.
It figures on the list of medical conditions that should prevent incarceration under Russian law, she said.