Bill Browder has described himself as "Putin's No 1 enemy". Now the Russian president had added weight to that claim by singling out the British investor at his controversial summit with US President Donald Trump.

The UK-based financier appeared to be part of what the US president called an "incredible offer" by Vladimir Putin to assist American investigators in their prosecution of 12 Russian intelligence officers accused of hacking crimes during the 2016 presidential election season.

"He offered to have the people working on the case come and work with their investigators with respect to the 12 people," Trump told reporters during a news conference in Helsinki following his joint summit with Putin.

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The special counsel investigating potential coordination between the Trump campaign and the Kremlin charged a dozen Russian military intelligence officers on Friday with hacking the Democratic National Committee and the Hillary Clinton campaign and then releasing the stolen communications online as part of a sweeping conspiracy to meddle in the election.

While Trump did not elaborate on the Russian leader's "incredible offer," Putin himself suggested that special counsel Robert Mueller could ask Russian law enforcement agencies to interrogate the suspects. He said US officials could request to be present at such questioning in line with a 1999 agreement on mutual legal assistance in criminal cases.

However, there was a catch: Russia would expect the US to return the favour and cooperate with interrogations of people "who have something to do with illegal actions on the territory of Russia". Putin highlighted the case of Bill Browder.

"No journalist had asked about me," Browder wrote in Time. "He just brought me up out of the blue ...To my mind, this can only mean that he is seriously rattled."

The American-born businessman, who has held British citizenship for the past two decades, was last year sentenced by a Russian court to nine years in prison on fraud and tax evasion charges.

More pertinently, he was also the driving force behind The Magnitsky Act, a 2012 US law targeting Russian officials over human rights abuses. It was named after Sergei Magnitsky, his lawyer whose investigations in 2008 uncovered a web of alleged tax fraud and corruption involving 23 companies and $230 million. He later died in Russian custody.

Russian President Vladimir Putin during a press conference with US President Donald Trump. Putin made an 'incredible offer' to assist in the Russian hacking case. Photo / AP
Russian President Vladimir Putin during a press conference with US President Donald Trump. Putin made an 'incredible offer' to assist in the Russian hacking case. Photo / AP

It seemed unlikely that American authorities would take seriously any such offer from Putin. But the moment underscored the impact Browder has had in his campaign against Russian corruption and rights abuses.

"It's a true affirmation of the fact that we've found Putin's Achilles' heel with the Magnitsky Act," Browder told the New York Times in an interview. "He's basically lost it, emotionally, because his own money in the West is now being seized under that Magnitsky Act."

Browder would not say where he currently was for reasons of safety. In March, the 54-year-old appeared before the Commons culture select committee and said Russia wanted him dead.

"I believe they want to kill me," he told MPs. "If they kill me in a very brazen way and don't get away with it there will be big repercussions. They haven't figured out a way yet where they can kill me and get away with it."

His remarks at the time came as police were investigating the nerve agent attack on former double agent Sergei Skripal and his daughter Yulia in Salisbury.

Browder ran a top investment fund in Russia in the 1990s but was deported in 2005 and his business seized.

In June 2007, Hermitage Capital Management, which was once Russia's largest foreign investor, was raided by police on tax avoidance charges.

Browder, the boss of Hermitage, called in his Moscow lawyers Firestone Duncan to check. One of those lawyers was Magnitsky, who concluded the allegations were baseless.

The Russian lawyer died in custody in 2009, officially from heart failure, but Browder has always claimed Magnitsky was murdered, and insists that he himself is a top Kremlin target.

Ever since Magnitsky's death, he has devoted his life to exposing corruption in a country he calls "a gangster state".

In May, he was detained in Spain on a Russian warrant before being released two hours later.

A Spanish police spokesperson said that Interpol had failed to delete the warrant from its database when it had expired, and that once at the police station, they had realised it was no longer valid.

"I live a life that is very different to most other people because all my actions are underlined by the assumption that the Russians are trying to kill me," he told the Telegraph in March.

"I'm pretty much used to the risk I'm taking. I'm not being emotional, nor am I exaggerating. It's a real situation."

Since the Magnitsky Act was passed in the US, six other countries or territories have taken similar measures, including Canada, Lithuania, Latvia, and Estonia.

Gibraltar was the latest country to pass the law in March, while the UK passed in February last year an amendment to the country's Criminal Finances Bill that was inspired by the Magnitsky Act. It allowed the government to freeze the assets of international human rights violators in the UK.

Browder, who says the Magnitsky measure are going "viral", said Putin had made the offer to the wrong leader.

"If he really wants me," he wrote, "he better go talk to Theresa May, who might have a few choice words for him after Russian agents spread the military-grade nerve agent Novichok across the cathedral town of Salisbury."

The Magnitsky Act

The Magnitsky Act is a US law signed by US President Barack Obama in December 2012 to bring sanctions against human rights abusers.

It was written in response to the incarceration and death of Russian lawyer Sergei Magnitsky.

Initially the law barred 18 Russian businessmen and government officials from entering the US and froze any assets they had in American banks.

The law was broadened out in 2016 and now applies sanctions to 44 suspected human rights abusers worldwide.

Similar bills have since been adopted by Canada and other European Countries. In 2017 the UK introduced the The Criminal Finances Bill which gives the government powers to seize the assets of foreign officials who abuse human rights.

In 2008 Magnitsky had untangled a complex web of tax fraud arrangements totalling £166 million ($320m), which was linked to individuals close to the Kremlin.

Magnitsky was then subject to investigations by the Russian government and eventually jailed without charge.

He was beaten to death in custody in murky circumstance days before he was due to be released. An independent investigation afterwards ruled that medical attention had been deliberately withheld from Magnitsky, leading to his death.