The UK government has imposed sanctions on those involved in the “deplorable” sentencing of the dual-national dissident Vladimir Kara-Murza after a Russian court dismissed his appeal against a 25-year sentence. Six figures – three judges, two prosecutors and an expert witness – will face sanctions for their role in a “politically motivated conviction”. Kara-Murza, who holds Russian and British citizenship, lost an appeal against his 25-year jail sentence on Monday, the RIA Novosti state news agency reported. The 41-year-old was jailed in April for treason and other offences. “This is desperate and unfounded,” tweeted the prime minister, Rishi Sunak. “Rejecting @vkaramurza’s appeal is unjustifiable. He should be released immediately.” The government said it had imposed asset freezes and travel bans on the Moscow city court judges Vitaly Belitsky and Ekaterina Dorokhina. Kara-Murza is one of a small number of prominent opposition figures who stayed in Russia and continued to speak out against Vladimir Putin after Moscow sent tens of thousands of troops into Ukraine in February 2022. The British government said it also imposed sanctions on Basmanny district court judge Natalia Dudar, who further extended Kara-Murza’s pre-trial detention, it said. “Kara-Murza, a dual British national, is being persecuted by the Russian regime for his anti-war stance,” the Foreign Office said in a statement. Others who were targeted in the sanctions include two prosecutors and an “expert witness” who Britain said had provided false justification for Kara-Murza’s detention. Kara-Murza was arrested two months after Russia invaded Ukraine, accused of spreading false information about the armed forces and declared a “foreign agent”. His detention came hours after CNN broadcast an interview with him in which he said Russia was run by “a regime of murderers”. Kara-Murza was later charged with treason over speeches he had made about the war, including one to the Arizona House of Representatives in March 2022 in which he said Putin was bombing Ukrainian homes, hospitals and schools.