Prime Minister Rishi Sunak called the decision by Moscow judges ‘unjustifiable’.
The UK has sanctioned six people following the “unjustifiable” decision to reject an appeal from a British-Russian dissident facing 25 years in jail for opposing Russia’s invasion of Ukraine.
Vladimir Kara-Murza was sentenced in April after being convicted of treason and spreading false information about the Russian army in a process he described as a “show trial”.
After Mr Kara-Murza’s appeal was rejected by judges in Moscow on Monday, Prime Minister Rishi Sunak called the decision “desperate and unfounded”.
He wrote on Twitter: “Rejecting @vkaramurza’s appeal is unjustifiable. He should be released immediately.
“The United Kingdom stands with him and his family.”
Kremlin critic Mr Kara-Murza, a close ally of murdered opposition figure Boris Nemtsov, has already survived two poisonings that left him in a coma in 2015 and 2017.
The UK Government confirmed it has sanctioned six individuals, including two Moscow city court judges, for their involvement in the conviction of Mr Kara-Murza.
They will be subject to travel bans and asset freezes.
The individuals were named as: Vitaly Alexsandrovich Belitsky and Ekaterina Mikhailovna Dorokhina, both Moscow city court judges; Natalia Nikolaevna Dudar, a Basmanny district court judge; Boris Georgievich Loktionov, the lead prosecutor in the trial; Danila Yurievich Mikheev, described as an expert witness for the Russian government; and Anna Evgenievna Potychko, a prosecutor.
Foreign Secretary James Cleverly said: “The rejection of Vladimir Kara-Murza’s appeal following his sentencing on bogus charges highlights the depravity of the Russian regime, and their complete disregard for human rights and freedom of expression.
“Today we’ve sanctioned six people connected with his case, sending a clear message that the UK will not stand for this treatment of one of its citizens.
“We will continue to support Mr Kara-Murza and his family, who have worked tirelessly to try and secure his release. I call on Russia to release him immediately and unconditionally.”
In April, the UK sanctioned five people – including two Russian spies – for their involvement in Mr Kara-Murza’s arrest.