Ukraine's top spy chief has claimed that Alexi Navalny may have 'died from a blood clot' amid widespread speculation that the outspoken Putin critic was murdered by Russian spies.

The anti-corruption campaigner, who was serving a 19 year prison sentence in the arctic Polar Wolf penal colony, was found dead earlier this month with Russia claiming he died form 'sudden death syndrome' while on a walk.

Yet many have speculated that he was killed on Putin's order, with Latvian President Edgar Rinkeish bluntly stating: 'Whatever you think about Alexi Navalny, as a politician he was just brutally murdered by the Kremlin.' 

Today, however, Kyrylo Budanov, the head of Ukraine's GUR military intelligence service, suggested his death could be down to natural causes. 

He told reporters: 'I may disappoint you, but what we know is that he really died from a blood clot. And this is more or less confirmed. This was not taken from the Internet, but, unfortunately, a natural [death].'

Prison authorities refused to release Nalvany's body for more than a week after he died. Yesterday, his corpse was finally handed back to his mother following an agonising nine-day wait. 

'Alexey's body was handed over to his mother. Many thanks to all those who demanded this with us,' Kira Yarmysh wrote on X.

She said mother Lyudmila Ivanovna, 69, was still in Salekhard, near the grisly IK-3 Arctic penal colony where her son had been held since December on 'extremism charges', and that the funeral is still 'pending'. 

'We do not know if the authorities will interfere to carry it out as the family wants and as Alexey deserves,' she added.

Following Navalny's death, his grieving widow Yulia Navalnaya accused Putin of 'torturing him dead'.

Ahead of the release of his body she said: 'Give us the body of my husband. You tortured him alive, and now you keep torturing him dead. You mock the remains of the dead.

'No true Christian could ever do what Putin is now doing with the body of Alexei.'

His mother had initially been blocked from seeing her sons remains, with reports claiming he had been left battered and bruised. She said that Russian officials were trying to blackmail her and threatened to 'do something' to the corpse. 

Following the release of his body, his widow called Putin 'Satan' for holding her husband's body 'hostage'.

The grieving widow said: 'What Putin is doing now is hate. No, it's not even hatred, it's Satanism.'

'What will you do with his corpse? How low will you sink to mock the man you murdered?' she continued.

The Kremlin has denied allegations of its involvement in Navalny's death, calling the West's reaction 'hysterical'. Putin, who famously never said Navalny's name in public, has not commented on the death of his most vocal critic.