"I'm deeply concerned for Mr. Kara-Murza - a British national imprisoned in Russia for speaking out against the invasion of Ukraine," Cameron said on social media platform X.

The comments follow news from Kara-Murza's wife, Evgenia, that he had left Omsk, where he had been imprisoned since September, and that she did not know where the authorities had taken him.

Cameron expressed solidarity with his wife and said he planned to meet her "soon".

Kara-Murza, who suffers from a nerve disorder after surviving two poison attacks, was jailed for 25 years last April for treason and spreading "false information" about Russia's war in Ukraine. He denied the charges and compared the proceedings to Stalinist show trials of the 1930s.

Russia's prison service did not respond to request for comment earlier on Monday on Kara-Murza's whereabouts.

(Reporting by Muvija M; Editing by Mark Porter and Mark Potter)