Kara-Murza, 42, serving 25 years for treason and other charges, has said the Russian security services have tried to assassinate him.

And concerns were raised for the health of one of President Vladimir Putin’s most vocal critics after he was transferred to a prison hospital last week.

His wife Evgenia Kara-Murza has said his lawyers were denied access to him for two days and she has had no information on his condition.

“It is absolutely unacceptable to deliberately prevent a family from knowing why their loved one is in hospital and what his current state of health is,” said Sunna Aevarsdottir, rapporteur of the Parliamentary Assembly of the Council of Europe’s .

“I am deeply concerned by this chilling act of inhumanity,” Aevarsdottir added, urging the Russian authorities to grant Kara-Murza’s lawyers access to him in hospital.

She noted that after the death of opposition leader Alexei Navalny in an Arctic prison colony in February, “there are widespread fears that the Russian regime is arranging for its most prominent critics to die behind bars”.

Kara-Murza campaigned against Putin for years and stayed inside the country to criticise the military offensive in Ukraine even as Moscow passed a raft of anti-dissent and military censorship laws.

Hundreds have been prosecuted under the measures and most Russian opposition figures not in jail have fled into exile.

The Kremlin critic suffers from a nerve disease after he fell severely ill in 2015 and 2017 in what he says were poisonings orchestrated by Russia’s FSB security service.

An investigation by several media outlets including Bellingcat, The Insider and Der Spiegel suggested that the FSB was involved in both cases.

Kara-Murza’s polyneuropathy “severely deteriorated during his detention and imprisonment”, Aevarsdottir said.

The PACE rapporteur urged Britain and the United States to inquire after Kara-Murza’s health and work to secure the release of the Russian-UK dual national.

In early May, Kara-Murza won a Pulitzer Prize for his dispatches from his prison cell, which the jury said were written at the risk of his life.

The Council of Europe is an international organisation that brings together the 46 signatory states of the European Convention on Human Rights.

Based in Strasbourg, its various bodies ensure that these rights are applied across the continent.

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