Mikhail Khodorkovsky, once Russia's wealthiest man, says President Vladimir Putin is "unpredictable" and "irrational."
The former head of energy giant Yukos lives in exile in Switzerland and alluded to French newspaper Le Monde that he was ready to "lead Russia".
Khodorkovsky was imprisoned for ten years on fraud charges after several very public clashes with Putin. In 2011, Amnesty International called the tycoon, who once had a net worth of £15bn according Forbes, a "prisoner of conscience". In 2013 he was pardoned by Putin on grounds that Khodorkovsky's mother had a terminal illness.
In an interview with Sobesednik, the political dissident said the end of the "Putin era" was close.
"Russia lives in long 15-16-year-long eras," Khodorkovsky said. "Putin's era, by my estimations, will be over by the end of 2015. The end of the era, of course, does not mean his swift exit! It will manifest itself in that he will cease to be constantly forgiven," he added.
"Putin will see out his current third term as president which runs out in 2018 and even run for reelection again," according to Khodorkovsky.
"It is assumed that Putin will stand in the election and win," he added. "To me it seems that the most possible scenario is a Putin victory in the 2018 elections and his departure within a year's time," he added.
He gave a full and frank critique of the Russian president's state of mind. "He lives in another reality—not an economic one, not a political one, but already, I think, in a religious one. He sees himself as the chieftain of the post-American world." He added that Putin's behaviour is "unpredictable" and "irrational."
The Russian exile was of the opinion that the possibility of nuclear war was small but if the US sent arms to Kiev, things would likely escalate.
Putin may begin full-scale war from Ukraine, the Baltic countries or the Middle East, the Russian opposition activist also said in the interview with the Russian weekly political newspaper.
"I believe, there are three places where everything can begin. Ukraine is the most obvious version, as it would be the continuation of a ground operation. The second variant is the Baltic States, where Putin continues his provocations, injecting and recriminations. And the third version is the Middle East - fallback option at all times," he said.
President Putin laughed off Khodorkovsky;s claims as a potential presidential rival, asking: "Which country is he going to be president of?"