Russian opposition leader Alexei Navalny has died in jail, the Yamal-Nenets autonomous district prison service confirmed on February 16.

"The inmate A.A.Navalny felt bad after a walk and almost immediately lost consciousness at the correctional colony No. 3 on February 16," Russia's prison service said in a statement.

"Medical staff arrived immediately, an ambulance was called. All necessary resuscitation efforts did not yield results," it added.

Navalny fell ill at around 1pm local time, prompting a call for an ambulance. Doctors attempted to resuscitate Alexei, but at 14:17 he died, according to reports on social media channels.

He was serving a 19-year prison sentence in the Correctional Facility No. 3 in the remote Khanty-Mansi Autonomous Okrug, in the “Polar Wolf” village of Kharp, where he was transferred from the Vladimir region at the end of last year.

The cause of death of Alexei Navalny is a detached blood clot. He felt unwell after the walk and lost consciousness. Doctors tried to resuscitate him but failed. He was 47 years old.

Presidential spokesman Dmitry Peskov commented on Navalny's death, stating that the causes should be determined by medical professionals. Peskov also reported that Russian President Vladimir Putin has already been informed of Navalny's death.

Russia’s most prominent opposition leader was arrested at the airport and then jailed in January 2021 after he returned to Russia from Germany, where he was being treated for poisoning with military-grade poison Novichok. German doctors later confirmed that the poison used against him was Novichok, which has been used in several other high-profile state-sponsored assassination attempts. In a bizarre twist, it emerged the poison was administered by smearing it inside a pair of blue underpants which briefly became a symbol of the Russian opposition.

Amnesty International recognised Navalny as a prisoner of conscience shortly after he was jailed.

He was quickly sentenced, and saw his sentence increased several times. His health had also visible deteriorated in the last year, with him showing considerable weight loss in his rare video addresses from jail at the various court sessions where he faced new charges.

The prison authorities claimed that he had not reported any previous health complaints. However, Navalny was reportedly close to death in April 2021 after he went on a hunger strike to protest against his jailing.

Following his jailing in 2021 there were two large country-wide protests as Russians were shocked at the heavy-handed way the state had treated him.

If reports of his death are confirmed, there is likely to be a similar short-lived protest but that is unlikely to turn into a revolution that could oust Russian President Vladimir Putin.

It will also hurt Putin at the polls in the upcoming presidential election slated for the middle of March.

The Kremlin is likely to flood the streets with riot police and ban any public gatherings if there are calls for new protests.

Missing

Navalny went missing for weeks in December, after he was moved to a new jail in the remote Russian hinterland, sparking rumours that he had died.

Indeed, the fear that he would not last to the end of his sentence have been dogging him since he was first jailed on charges of breaking his parole arrangement, while he was hospitalised in Germany.

The head of Navalny's anti-corruption watchdog FBK Ivan Zhdanov said at the time that Navalny was "most likely being transferred" to a different prison, but also stressed his team knew "nothing" about the politician's fate. Activists grew increasingly worried about his fate, especially with Navalny appearing ill on camera just days before his disappearance in early December.

A leading opposition figure, Navalny made a name for himself by publicly calling out graft at the top of the Kremlin hierarchy and his FBK regularly embarrassed Russia’s top officials with in-depth investigations into their wealth in a series of YouTube video exposes that garnered millions of views.

In one expose he linked Putin to a $700mn superyacht with six decks, two helipads, a salon-spa, saunas and a swimming pool with a retractable cover which doubles as a dance floor. In another video an investigation into the so-called “Putin's Palace” was seen by over 50mn people, just two days after its release.

He founded the FBK which had become a political organisation attempting to contest regional elections until the Kremlin banned it completely shortly after Navalny was jailed. The regional office network was closed and most of the other top FBK officials then fled into exile.

Navalny advocated a strategy of “smart voting” where opposition activists rally around any candidate in any election that stood a chance of defeating a Kremlin-sponsored incumbent, simply to spoil the Kremlin’s ability to control the political process, and the tactic scored several successes by upending several regional votes.

Navalny was one of the leaders of the large-scale anti-government protests in 2011-2013, which culminated in his running for the mayor of Moscow in 2013, where he did surprisingly well. He has called Putin a "madman" and labelled most of government’s top officials “crooks and thieves” – a slur that stuck.