The UK Foreign Secretary has called for the immediate release of Russian dissident Vladimir Kara-Murza on the second anniversary of his imprisonment.

Lord David Cameron condemned the Kremlin’s “callous disregard” for the opposition figure’s declining health as he languishes in jail on “fabricated” charges following his criticism of the war in Ukraine.

Dual British-Russian citizen Mr Kara-Murza, a journalist and opposition activist, was imprisoned in April 2022 and convicted of treason last year.

He is serving 25 years – the harshest sentence handed down to a Kremlin critic in modern Russia.

But he is among a growing number of dissidents held in increasingly severe conditions under President Vladimir Putin’s political crackdown.

Lord Cameron attacked Russia on Thursday for subjecting Mr Kara-Murza to “degrading and inhumane conditions in prison, clearly designed to further damage his physical and mental wellbeing”.

“Two years on from Vladimir Kara-Murza’s arrest on fabricated charges, I urge the Russian authorities to release him immediately on humanitarian grounds,” he said in a statement.

“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.

“Putin locked him up in a bid to silence him.”

He added: “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.

“Russia’s depraved treatment of political prisoners must end.”

Mr Kara-Murza, who twice survived poisonings that he blamed on Russian authorities, has rejected the charges against him as punishment for standing up to Mr Putin.

He likened the proceedings to the show trials under Soviet dictator Joseph Stalin.

Meanwhile, members of the US Congress from across the aisle were joined by Mr Kara-Murza’s wife, Evgenia Kara-Murza, as they renewed calls for his release at an event at Capitol Hill on Thursday.

Ms Kara-Murza, who lives in the US with their three children, told a crowd: “I want to thank each and every one of you here for joining with me in my fight, not just for Vladimir’s freedom, but truly for his life.”

Ms Kara-Murza has said her husband has spent months in solitary confinement, a punishment that has become common for Kremlin critics and is widely viewed as a means of placing added pressure on them.