Mr Khodorkovsky says he knows he is taking a risk every time he speaks out against the Russian leader but claims that the practice is rife and has to be stopped.

Speaking to Sky News, the former head of oil giant Yukos, who was imprisoned for eight years by Mr Putin's government for tax evasion and theft in a controversial case, said it was "really important to fight this".

He explained: "Putin's clique use money in order to illegally put pressure on the British political system. And this is happening - directly, indirectly, via legal firms, via lobbying businesses, via directly influencing politicians and so on and so forth.

"This is the most dangerous aspect and it's really important to fight this.

"It is clear that the assets of his cronies held on enemy soil - and he considers the UK his enemy - are a sign of betrayal, unless they are used to influence British politics."

Tensions between Russia and the West are at their highest in years, following a Russian troop buildup near its border with Ukraine sparking fears Moscow could launch an invasion.

Putin is 'in fact afraid' of a war

The UK and its NATO partners have repeatedly warned Russia against further aggressive moves against Ukraine.

The Kremlin denies it is planning to attack and argues NATO support for Ukraine - including increased weapons supplies and military training - actually constitutes a growing threat to Russia.

Mr Khodorkovsky, who moved to the UK after he served his prison sentence and was subsequently pardoned by Mr Putin, said the more people who serve the Russian president and hold assets in London, the more they will try to influence the system to stop the UK from assisting Ukraine.

He describes Mr Putin as being "capable of all sorts of actions, including ill-judged ones" but perhaps surprisingly, also says he believes he is "in fact afraid" of a war.

"At present the opposition in Russia has been reduced to a bare minimum - by way of arrests, pressure on their families etc.

"But it also means that opposition sentiments have gone deeper underground. This means that Putin doesn't really know what society's reaction is going to be to this or that action.

"It is one of the reasons why he is in fact afraid of starting a war with Ukraine.

"He is not confident that Russian society would support such a war - and I'm sure that Russian people will not support it.

"But he doesn't even know how strong the backlash would be. And that backlash could be very powerful."

To say 'let's just pull back' would mean a display weakness

He went on: "One needs to appreciate that Russia and Ukraine are very close - not just geographically, but because of family ties.

"Thirty million Russian families are either mixed marriages or have relatives in Ukraine or have Ukrainian roots.

"This is a serious challenge for Putin - it is something that he is afraid of tackling.

"On the other hand he is also aware that he has raised the stakes so high, that to simply say 'let's just pull back' would mean to display his weakness. And that would be a huge risk for him."

And he warned that if in the past Mr Putin might have been a man who would listen to others, or exercise some caution, "this has changed".

He added: "He is convinced of his own infallibility, and he is capable of all sorts of actions, including ill-judged ones."