New 'leaked' evidence shows Vladimir Putin's most charismatic political foe was 'poisoned to death' in his hellhole Arctic jail.

Alexei Navalny, 47, died in February and was widely seen to have been killed on the orders of the Kremlin autocrat.

The new evidence indicates poisoning is likely the way his alleged assassination was carried out.

The suspicion is that Russian hitmen used Novichok - the deadly nerve agent they earlier failed to kill him with.

The Insider independent news outlet disclosed documentary evidence that Navalny had complained to warders of a 'sharp pain in the stomach' while locked up as a political prisoner in Polar Wolf jail.

Russia has officially claimed the case 'does not have a criminal nature' and that that Putin foe and anti-corruption campaigner died of 'natural causes'.

But The Insider has revealed an alleged cover-up over the agonising symptoms Navalny reported in his last hours.

The sensational documents claim Navalny 'felt a sharp deterioration of his health condition while in the exercise yard' on February 16.

One day earlier, a video showed him looking and sounding healthy despite his incarceration - as he mocked the judge in charge of his latest bogus trial.

After reporting acute abdominal pain, Navalny was taken from the exercise yard, the leaked documents claim. 

'Navalny lay down on the floor and began to complain of sharp pain in the abdominal area,' one leaked document states. 

'He started reflexive ejection of his stomach contents, had convulsions, and lost consciousness, which was immediately reported to the medical staff of the correctional facility.'

This was 'consistent with those that would have been expected had Navalny been poisoned', said the report.

Yet later versions of the report removed 'all mention of abdominal pain, vomiting, and convulsions', indicating censorship by Putin's regime.

Navalny's wife had earlier insisted that 'in the last minutes before his death, [Alexei] complained of acute pain in his stomach.' The new leaked documents indicate she was right.

If poisoning is correct, it is not the first time that Putin has sought to assassinate a politician who relentlessly mocked him.

Earlier, a botched bid to kill him with radioactive nerve agent Novichok sprinkled in his underpants had failed.

A medic who had treated Navalny, Alexander Polupan, said of the latest disclosures: 'The official cause of death - a heart rhythm disorder - would in no way explain the symptoms….sharp abdominal pain, vomiting, or seizures.

'These symptoms can hardly be explained by anything other than poisoning.

'The short interval between the abdominal pain and the convulsions suggests the possibility of exposure to an organophosphorus agent, for instance - the same class of substances as Novichok, but in this case it may have been applied internally.'

Putin's first reaction on hearing news of Navalny's tragic death was to smile.

Navalny's widow has accused Putin of being her husband's 'murderer'.

She raged: 'His place is in prison, and not somewhere in The Hague, in a cozy cell with a TV, but in Russia - in the same colony and the same 2-by-3-metre cell in which he killed Alexei.'