Alexi Navalny – nemesis of Vladimir Putin and his regime – has died in prison.

The outspoken opposition leader was “taken ill” after a walk.

Prison authorities said Navalny quickly lost consciousness.

He died soon afterwards in the brutal penal colony of Yamal, in western Siberia – some 125 miles inside the Arctic Circle.

The Russian news agency Interfax said doctors spent 30 minutes trying to resuscitate Navalny.

The state press service Tass said the cause of death is “being established”.

Antony Blinken, the US secretary of state, said Russia was “responsible” for the dissident’s death.

In 2021 American President Joe Biden warned the consequences of Navalny’s death “would be devastating for Russia”.

Navalny, 47, was a prominent opposition politician in Russia.

He led street protests about corruption with Putin’s regime.

Lord Cameron, Britain’s Foreign Secretary, said:

“Putin should be accountable for what has happened – no one should doubt the dreadful nature of his regime.”

Rishi Sunak described Navalny’s death as a “huge tragedy” for “the people of Russia”.

He added:

“As the fiercest advocate for Russian democracy, Alexei Navalny demonstrated incredible courage throughout his life.

Navalny was jailed for two-and-a-half years in 2021 after returning to Russia upon his recovery from the attempted poisoning.

Nine years were added to the sentence in March 2022 on embezzlement and contempt of court charges.

A court then extended his sentence to 19 years in August last year on extremism charges and moved him to a harsher prison.