Reuters photographer Putin and Pugachev, photo dated July 28, 2000.
Sergei Pugachev told independent Russian TV channel "Dozhd" ("Rain") that he is in the process of suing Russia for $15 billion over the state's appropriation of his assets.
(As a reference point, $15 billion is roughly equal to the GDP of Senegal.)
Pugachev also told Dozhd that he wrote to Putin last year telling the Russian president of his intention to do so.
Exact details about the case are unclear at the moment.
Pugachev was a big player in Moscow during Putin's first two terms. He founded Mezhpromback (International Industrial Bank) in Moscow in 1992, and within four years was a "Kremlin powerbroker," helping politicians win elections and speaking to Putin "almost every day probably."
But in Russia's inner financial circles, nothing ever lasts. Relations soured in 2010, and he ultimately fled to London in 2011.
Most recently, Pugachev told the Financial Times that he is representing himself in another legal battle (Russia's state deposit insurance agency, the DIA, accused him of being behind the $2 billion shortfall at Mezhprombank). He said that he only has £45,000 (about $70,200) left in accounts and can't afford lawyers.
He even claimed that he was actually the one who suggested professional spy Vladimir Putin as a candidate - not Yeltsin-era Boris Berezovksy, who had been widely credited with anointing Putin.
Pugachev did concede that Putin was already an "insider" by that time, although he doesn't think Putin's plan led to the presidency.
"Putin is not someone who sets strategic plans; he lives today." Pugachev told Time last October. "He had no plans; he didn't aim to become president. He hadn't thought of that. He didn't plan to remain in the government at all."