The British government said it had summoned diplomats from the Russian embassy "to make clear that we hold the Russian authorities fully responsible" for the death of Kremlin critic Alexei Navalny.

In its statement, the Foreign Office said Navalny's death in an Arctic prison "must be investigated fully and transparently". "We stand today with his family, friends, colleagues and supporters. Our thoughts go out to them," a UK Foreign Office spokesperson said.

Navalny died on Friday in the Arctic penal colony where he was serving a 19-year sentence, Russia’s prison agency said. He was 47.

Russia's Federal Penitentiary Service reported Navalny felt sick after a walk on Friday and lost consciousness at the penal colony in the town of Kharp, in the Yamalo-Nenets region about 1,900 kilometers (1,200 miles) northeast of Moscow.

An ambulance arrived, but he couldn't be revived; the cause of death is "being established,” it said. Russia’s main state TV channel interrupted its newscast to announce the death.

Navalny had been jailed since January 2021, when he returned to Moscow to face arrest after recuperating in Germany from nerve agent poisoning. He was later convicted three times.

Born in Butyn, about 40 kilometers (25 miles) outside Moscow, Navalny received a law degree from People’s Friendship University in 1998 and did a fellowship at Yale in 2010.