Speaking to The Independent, Bill Browder – a Putin critic who has felt the threat from the Kremlin himself – said: “Let’s make no mistake, Putin assassinated Alexei Navalny.

“He did so because Alexei Navalny was brave enough to stand up to Putin. He did so because Navalny offered the Russian people and alternative to kleptocracy and repression. This is a tragic day for Navalny and his family but also for Russia and the hope for a better future.”

Rishi Sunak described the reported death of Alexei Navalny as “terrible news”, adding: “As the fiercest advocate for Russian democracy, Alexei Navalny demonstrated incredible courage throughout his life.”

Nato Secretary-General Jens Stoltenberg said he was “deeply saddened and disturbed” by the news.

“We need to establish all the facts, and Russia needs to answer all the serious questions about the circumstances of his death,” he warned.

Ukrainian leader Volodymyr Zelensky went further, saying it was “obvious” Vladimir Putin was directly behind the death.

Our politics and Whitehall editor Kate Devlin has full report here:

Responding to the Russian authorities’ announcement that Alexei Navalny has died in prison, the Kremlin critic’s chief of staff said: “We have no reason to believe state propaganda.

“If this is true, then it is not ‘Navalny died’, but ‘Putin killed Navalny’ and only that.

“But I don't trust them one penny.”

And his spokesperson Kira Yarmysh said: “We don't have any confirmation of this yet. Alexei’s lawyer is now flying to Kharp. As soon as we have any information, we will report it.”

Exiled Russian businessman and Kremlin critic Mikhail Khodorkovsky has said that Vladimir Putin is responsible if Alexei Navalny is dead.

The Kremlin has said it has no information about Alexei Navalny’s cause of death, but said that Russian president Vladimir Putin was aware.

Kremlin spokesperson Dmitry Peskov said Russia’s penitentiary service was making all checks regarding Mr Navalny, but that he had no additional information.

Russia’s prison agency said Mr Navalny had died in prison aged 47. The federal prison service said in a statement that Mr Navalny felt unwell after a walk on Friday and lost consciousness. An ambulance arrived to try to rehabilitate him, but he died.

There was no immediate confirmation of Mr Navalny’s death from his team.

My colleague Athena Stavrou has more in this breaking story, which you can refresh for updates:

Alexei Navalny was “brutally murdered by the Kremlin”, Latvian president Edgars Rinkevics has said.

“Whatever your thoughts about Alexei Navalny as the politician, he was just brutally murdered by the Kremlin. That’s a fact and that is something one should know about the true nature of Russia’s current regime. My condolences to the family and friends,” he wrote on X/Twitter.

The EU holds Vladimir Putin’s regime “solely responsible” for Alexei Navalny’s death, European Council president Charles Michel has said.

Alexei Navalny foresaw the inevitable retribution he would face for speaking out about corruption in Vladimir Putin’s Russia, saying: “I am in the very blackest part of the black list.”

Over the years, Mr Navalny faced physical attacks, repeated arrests, investigations, and criminal proceedings. The assassination attempt, via Novichok poisoning in Russia resulted in him being evacuated to Germany in a coma for life-saving treatment.

His family and lawyers have previously said he had been suffering from an acute, undiagnosed, stomach illness while in prison. That, and general deprivation, has led to alarming weight loss and fainting spells.

He had returned to Russia following the poisoning despite warnings from allies and friends that it would be highly dangerous. He was arrested at the airport, put before a court, and sentenced to two and half years of a former suspended jail term for alleged fraud.

This was just a holding move by the government prosecutors. In August 2022 he was sentenced to nine years in prison after being found guilty of spending public donations to his Anti-Corruption Foundation on “extremism and personal needs”. In August this year, he was sentenced to a further 19 years of a raft of “extremism” charges. There were further charges, this time of “vandalism”, due.