The billionaire, whose name has been withheld for his safety, was allegedly secretly taped in mid-March expressing his contempt for the 'crazy' Russian president's invasion of Ukraine.

In the recording, obtained by New Lines Magazine, he said: '[Putin] absolutely ruined Russia's economy, Ukraine's economy and many other economies — ruined [them] absolutely.

'The problem is with his head. … One crazy guy can turn the world upside down.'

The oligarch was said to be among Russia's 200 richest people and currently residing outside of the country.

He was part of a 'close circle of 20 or 30 people' whom Putin met with in the build-up to his illegal annexation of Ukraine's Crimean peninsula in 2014, a former European security chief told the magazine.

The businessman reportedly spent much of the 11-minute taped conversation asking his colleague, a high-flying investor, how he could protect his investments in Europe from Western sanctions.

But he went on to blast Putin for the false pretense of trying to rid Ukraine of 'Nazis and fascists', adding that 'we all hope' he dies from his cancer or an internal coup.

The businessman reportedly continued: 'It's unbelievable. For what? He killed more people than in 10 years in the [Soviet] Afghan war.'

Russia's secret service is said to have circulated a memo to regional chiefs around the time of the recording ordering them to ignore rumours about the Putin's health.