Russian authorities have refused to open a criminal investigation into the death of opposition leader Alexei Navalny - who died in an Arctic penal colony, his widow has said.

Mr Navalny, the fiercest political foe of Russian President Vladimir Putin, died in February in the remote northern prison colony while serving a 19-year sentence on extremism charges that he had condemned as politically motivated.

Authorities said he became ill after a walk but have otherwise given no details on Mr Navalny's death.

Russian officials only said that Mr Navalny died of natural causes shortly after taking a walk and have vehemently denied involvement both in an attempted poisoning claim and in his death.

In a video published on social media, Yulia Navalnaya, Mr Navalny's widow, said she received a letter from Russian investigators detailing the cause of her husband's death.

In the video, Ms Navalnaya said she was told Mr Navalny died from a combination of 'a dozen different diseases'.

Investigators told her Mr Navalny finally died from arrhythmia, or an irregular heartbeat.

Ms Navalnaya disputed Russian officials' version of events and said her husband exhibited no instances of heart disease while alive.

The letter, Ms Navalnaya said, was 'a lie'.

'They are hiding what really happened that day ... when Alexei became ill, he was not taken to the medical unit, but back to the punishment cell.'

There, she said, Mr Navalny was 'dying alone'.

The document said officials would not open a criminal investigation into the opposition leader's death because no offence had been committed, Ms Navalnaya said.

'As long as Putin is in power, there will be no investigation,' Ms Navalnaya said, vowing to continue to probe the circumstances around her husband's death.

An investigation into Navalny's death by Russian human rights organisation gulagu.net alleged a forensic expert who examined the corpse of the 47-year-old opposition leader has come under secret services pressure to hide his findings.

The initial Russian reports say Navalny died of a 'detached blood clot' in a hellhole Arctic prison - dubbed Polar Wolf - where he had been jailed in an act of political repression.  

The organisation stated: 'The forensic expert was pressured to conceal the discovery of blood clots….in the calf muscles and arms'.

This indicated 'the formation of blood clots as a result of disruption of normal blood circulation'.

A source said that the forensic expert 'was inclined to believe that four to five hours before the onset of biological death, the prisoner was tied up and his arms and legs'.

His limbs 'were tightly tied just so that the blood stagnated and blood clots formed in it, which then clogged the pulmonary artery and blood vessels of the brain', said a statement from the organisation, headed by Vladimir Osechkin.

No proof was offered for the allegation of direct physical abuse but Western leaders and other Russian opposition figures have already alleged Navalny was murdered on Vladimir Putin's orders.