Our correspondent Diana Magnay is giving her immediate reaction to the news now.
She says this is an "absolute tragedy" and a "stain on the conscience of the Russian state".
Alexei Navalny has been "deteriorating slowly" and held in solitary confinement for much of the time since we was imprisoned in Russia.
But she says it "does seem very shocking that his health should have deteriorated to the extent that he's dead".
Mr Navalny has been seen in court videos recently, where he "constantly seemed upbeat and cheerful", and had a lot of social media presence.
