Kara-Murza, who holds Russian and British citizenship, lost an appeal against his 25-year jail sentence on Monday, the RIA state news agency reported.
"This is desperate and unfounded," Prime Minister Rishi Sunak said on X, the social media platform previously known as Twitter.
Britain said it has imposed asset freezes and travel bans on Moscow City Court Judges Vitaly Belitsky and Ekaterina Mikhailovna Dorokhina.
"Kara-Murza, a dual British national, is being persecuted by the Russian regime for his anti-war stance," Britain's Foreign, Commonwealth & Development Office said in a statement.
Others who were targeted in the sanctions include two prosecutors and an "expert witness" whom Britain said had provided false justification for Kara-Murza’s detention.
Persons:
Murza, Sunak, Vladimir Kara, Rishi Sunak, Vitaly Belitsky, Ekaterina Mikhailovna Dorokhina, Vladimir Putin, Natalia Nikolaevna Dudar, Murza's, Kara, Britain, Putin, Muvija, William James, Leslie Adler
Organizations:
Russian, Commonwealth & Development, CNN, Representatives, Thomson
Locations:
Britain, Moscow, Russia, Ukraine, Basmanny, British, Arizona