Britain on Friday sanctioned a Russian judge and four others linked to the arrest and alleged poisonings of Kremlin critic Vladimir Kara-Murza who was jailed for 25 years this week for treason and other offences.
Russia branded the moves as illegal and former president Dmitry Medvedev reiterated a threat, frequently voiced by hardline nationalist Russian commentators, that Moscow's nuclear weapons could sink Britain into the sea.
Kara-Murza, a 41-year-old opposition politician who holds both Russian and British passports, spoke out against President Vladimir Putin for years and had successfully lobbied Western governments to impose sanctions on Moscow and individual Russians for purported human rights violations.
The sentence imposed on Monday at his closed hearing, which Kara-Murza compared to the Stalinist show trials of the 1930s, was three times longer than any previously imposed for criticising Russia's war in Ukraine, and formed part of a new wave of repression in the past few weeks.
"Russia's treatment and conviction of Vladimir Kara-Murza once again demonstrates its utter contempt for basic human rights," British foreign minister James Cleverly said.
"The UK will continue to support Mr Kara-Murza and his family. I call on Russia to release him immediately and unconditionally."
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Britain imposed travel bans and asset freezes on Elena Lenskaya, a judge who it said had approved Kara-Murza's arrest, and two investigators involved in the arrest - Denis Kolesnikov and Andrei Zadachin.
Twice, in 2015 and 2017, Kara-Murza fell suddenly ill in what he said were poisonings by the Russian security services.
The new British sanctions also target two agents at Russia's FSB security service - Alexander Samofal and Konstantin Kudryavtsev - who Britain said were involved in the poisonings. Russian authorities have denied involvement in any such attacks.
State news agency TASS quoted the Russian embassy in London as saying the sanctions were a hostile and unjustified step.
Medvedev, deputy chairman of Putin's Security Council, said Russia could not care less about decisions by Britain.
"Britain was, is and will be our eternal enemy. At least until their impudent and disgustingly damp island disappears into the abyss of the sea from the wave created by the latest Russian weapons system," he wrote on Telegram.
Medvedev, once seen as a Western-leaning moderniser, has turned himself into an arch-hawk since Russia invaded Ukraine last year, frequently brandishing nuclear threats and warning of the risk of a global "apocalypse".
(Reporting by Muvija M, writing by Sachin Ravikumar and Mark TrevelyanEditing by Gareth Jones)
