Cameron promises to hold those responsible accountable, as Navalny’s mother takes legal action to recover his body.

The United Kingdom has imposed sanctions on six officials overseeing the Arctic penal colony where Russian opposition leader Alexey Navalny died.

The sanctions, announced by British Foreign Secretary David Cameron on Wednesday, target the head and five deputy heads of the IK-3 penal colony in Kharp in Russia’s Yamal-Nenets region, who now face a ban on entering the UK and will have their assets frozen.

“Those responsible for Navalny’s brutal treatment should be under no illusion – we will hold them accountable,” Cameron said.

“It’s clear that the Russian authorities saw Navalny as a threat and they tried repeatedly to silence him.”

The UK is the first country to impose sanctions in response to Navalny’s death, the Foreign Office said as it described him as “a political prisoner who dedicated his life to exposing the corruption of the Russian system, calling for free and open politics, and holding the Kremlin to account”.

The sanctioned individuals include Vadim Konstantinovich Kalinin, who oversaw the prison where Navalny was kept in solitary confinement for up to two weeks at a time, it added.

The penal colony about 1,900km (1,200 miles) northeast of Moscow is considered one of the toughest in Russia. Its inmates are convicted of grave crimes.

Russian authorities have said the cause of 47-year-old Navalny’s sudden death on February 16 at the colony, called “Polar Wolf”, is still unknown and have refused to release his body for the next two weeks, pending a preliminary inquest.

On Wednesday, Navalny’s mother, Lyudmila Navalnaya, filed a lawsuit at a court in the town of Salekhard near the penal colony, contesting officials’ refusal to release her son’s body so she could bury him with dignity.

A closed-door hearing has been scheduled for March 4, Russia’s state news agency TASS reported.

Navalny’s widow, Yulia Navalnaya, released a video on Monday claiming the authorities had not yet handed over his body because they were waiting for traces of the Novichok nerve agent to disappear.

Kremlin spokesperson Dmitry Peskov has rejected allegations of a cover-up and President Vladimir Putin’s alleged involvement as “unfounded, insolent accusations about the head of the Russian state”.

The UK’s decision to take targeted punitive action against the prison heads coincided with plans by the United States to impose sanctions against Russia over the Putin critic’s death and the two-year war in Ukraine, set to be announced on Friday.

Cameron will attend the G20 foreign ministers’ meeting in Brazil later on Wednesday and the Foreign Office said “he will use the opportunity to call out Russia’s aggression and its global impact directly to Russian Foreign Minister Sergey Lavrov.”