The man who helped blow the whistle on rampant corruption in Moscow and has been described as Vladimir Putin's "number one enemy" has blasted Donald Trump's meeting with the Russian President, saying Mr Putin should be "severely contained, not engaged".
Bill Browder — who is the CEO of Hermitage Capital Management — once ran Russia's most successful investment fund, but in 2005 he was declared an enemy of the state for his part in exposing massive corruption to the tune of more than $300 million.
Now Mr Browder — who is a lobbyist for the Magnitsky Act, a law that punishes Russian human rights violators — has found himself at the centre of negotiations between Mr Trump and Vladimir Putin at their Helsinki summit.
Mr Trump said Mr Putin privately made an "incredible offer" to help American investigators in their prosecution of 12 Russian intelligence officers accused of hacking crimes during the 2016 presidential election season.
In return, the Russian President said he expected the US to help with investigations into Mr Browder.
"There is a well-known case of Hermitage Capital. According to our investigation, Browder's partners have illegally made more than $1.5 billion in Russia," Mr Putin said.
"They have paid no taxes in Russia and the United States but they transferred this money to the United States, 400 million [dollars] has been channelled into [Hillary] Clinton's pre-election campaign."
Mr Browder, who has written a book on his fight for justice and how he routinely deals with Russian attempts to have him arrested through Interpol, spoke to the ABC's The World program ahead of the Trump-Putin summit.
Speaking on condition his location wasn't revealed, Mr Browder explained why the Helsinki meeting gave credibility to Mr Putin, among other issues.
'This meeting should not be happening in first place'
"Russia is a country with an economy the size of the state of New York, the military budget is 90 per cent less than the United States' military budget, Vladimir Putin is effectively a war criminal, he's been out and about bombing innocent women and children in Syria, taking over large swathes of Ukraine, knocking passenger planes out of the sky with civilians on board.
"He's not a guy who Donald Trump, the leader of the free world, should be meeting with on par in Helsinki. Just by having the meeting it's an enormous, unjustified gift to Vladimir Putin."
Putin needs to be severely contained not engaged
"If the US was doing the right thing right now, the only approach to Vladimir Putin is severe containment … like we did during the Soviet Union.
"Instead of having a communist ideology, we have a criminal ideology, and the world, the United States and other countries, shouldn't give Putin an inch.
"This is a guy who should be contained, not engaged.
"Russia is a malign country doing terrible things all around the world. They're meddling in elections in the United States, in Britain, in France, Germany, Italy, Spain, they're invading foreign countries, they're cheating in the Olympics.
"This is a country which is effectively a criminal state run by a criminal, Vladimir Putin. And to view them as anything other than that basically is a huge injustice to all the victims all over the world of Vladimir Putin and his corrupt regime."
Presidents have done this before and it's been a disaster
"Donald Trump is not the first world leader to try to do this. George W Bush had a meeting with Vladimir Putin in 2001, looked into his eyes and saw his soul, and then the next 35 meetings didn't go so well.
"[Barack] Obama wanted to reset relations with Russia, that was a total disaster.
"And so, you have this strange sort of arrogance of a US President wanting to go in and somehow taming Vladimir Putin.
"Vladimir Putin has no intention of becoming a sort of sensible participant in world affairs. He's a malign influence in world affairs."
'Digital Pearl Harbor': Russian agents' indictment is 'damning'
"Basically what it shows is it's like a digital Pearl Harbor.
"The Russian government, Russian military intelligence, got involved in the most heinous way in the US elections and tried to sway the outcome. And that can't be allowed to stand."