The key Putin critic is currently serving a sentence of more than 30 years at a penal colony east of Moscow (Picture: AP)

Aides to prominent Russian opposition figure Alexei Navalny have expressed grave concern over his health after he reportedly collapsed in prison.

Navalny, 47, is currently serving a sentence of more than 30 years at a penal colony east of the Russian capital of Moscow on what the key Kremlin critic claims are trumped up charges. 

Close associates of the politician, journalist and activist say they have been unable to make contact with him for more than three days. Navalny has also not appeared at recently scheduled hearings concerning his case.

The worrying update comes as Putin announced he will run again in the upcoming presidential elections and aim to stay in the Kremlin for six more years.

Maria Pevchikh, chair of the opposition figure’s Anti-Corruption Foundation, wrote on X, formerly Twitter: ‘We have learned that last week [Navalny] had a serious health-related incident.

‘Navalny’s life is at great risk. He is in complete isolation right now.’

Kira Yarmysh, another Navalny spokesperson, added the Putin critic had recently been put on a drip by prison staff, having suffered a spell of dizziness that saw him have to lie down on the floor of his cell.

Navalny has reportedly been subject to abuse and long periods of solitary isolation at the facility where he is currently being held (Picture: AP)

Writing on X, she described this as closely resembling a ‘hunger faint.’

She went on to say: ‘Now it’s the third day that we don’t know where he is. Before that, there were at least occasional letters from him, albeit censored ones, but there have been no letters all week.’

Though Navalny survived a poisoning attempt, conducted by use of a highly toxic nerve agent, back in 2020, he has since suffered from long-term health issues.

The prominent Russian opposition leader was convicted in August on charges of ‘extremism’ (Picture: AP)

Russian President Vladimir Putin has consistently denied any attempt on the opposition figure’s life.

His government has additionally refused to provide any public comment on Navalny’s case, insisting he is being treated in much the same way as any other member of Russia’s 433,000-strong prison population.

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The facility where Navalny is currently being held, which falls into the category of a ‘special regime colony’, is typically reserved for ‘particularly dangerous repeat offenders.’

This represents prisoners convicted of the most serious crimes, such as murder, rape, drug trafficking, theft or robbery.

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The key Putin critic is currently serving a sentence of more than 30 years at a penal colony east of Moscow.