UK Foreign Secretary David Cameron accused the Kremlin Thursday of subjecting one of its biggest critics, Russian-British citizen Vladimir Kara-Murz, to "degrading and inhumane conditions" since his arrest two years ago.
Kara-Murza, one of the most prominent opponents of President Vladimir Putin, was sentenced to 25 years in prison last year -- the longest known sentence for a Russian dissident in recent times.
He 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.
In a statement on the second anniversary of Karza-Murza's detention, Cameron called the charges against him "fabricated" and urged Moscow to release him immediately on humanitarian grounds.
"A committed human rights activist striving for a democratic Russia, and an outspoken critic of the war in Ukraine, Mr Kara-Murza was considered a threat by the Kremlin," Britain's top diplomat said. "Putin locked him up in a bid to silence him."
Accusing Russian authorities of "callous disregard for his declining health", Cameron added the priohe was "now being subjected to degrading and inhumane conditions in prison, clearly designed to further damage his physical and mental well-being".
The former prime minister said Kara-Murza has been refused much-needed urgent medical treatment, and vowed to keep publicising his plight.
"Through diplomatic interventions at the highest levels, financial sanctions targeted at those behind his poisoning and imprisonment, and by raising his case on the international stage, we are sending a clear message that the UK will not stand for this abhorrent treatment of one of our citizens," he added.
"Russia's depraved treatment of political prisoners must end."
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 last month 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.
Moscow's crackdown on critics has worsened since its invasion of Ukraine in February 2022, with Russia currently counting nearly 700 political prisoners, according to Nobel Prize-winning rights group Memorial.