Russian authorities have announced new charges against the US-born British financier William Browder – another twist in a highly politicised case that has seen the Kremlin tirelessly go after one of its most vocal international critics.

Bill Browder: I found a way to challenge Putin. He's really sore about it Read more

On Monday, prosecutors in Moscow accused Browder of organising a “transnational criminal group” and claimed he may have poisoned Sergei Magnitsky, the lawyer who uncovered huge government-linked fraud and died in prison. Browder has accused Russian prison officials of causing Magnitsky’s death. Prosecutors said the charges carried a potential 20-year sentence.

The move would increase unease about the prospect of a Russian candidate taking over Interpol, the international police agency. The Sunday Times reported that British officials believe Alexander Prokopchuk, a 56-year-old veteran of the Russian interior ministry, was likely to be elected the next secretary general of the organisation during its general assembly this week. It comes after Interpol’s Chinese head Meng Hongwei resigned in October. China’s anti-corruption authorities have been investigating him.

Russia has made a habit of using Interpol’s “red notice” international arrest warrants in cases Kremlin critics believe to be politicised, including on multiple occasions against Browder. Earlier this year, Browder was detained by Spanish authorities, apparently on an Interpol warrant requested by Russia. He was released after a squall of publicity and complaints from supporters.

On Sunday, Browder tweeted a photograph of the Interpol general assembly and said: “Here’s the room where Putin will attempt his most audacious operation yet: to take over Interpol so he can expand his criminal tentacles to every corner of the globe.”

Facebook Twitter Pinterest The Russian prosecutor general’s office spokesman, Alexander Kurennoi (left), after a press briefing about the charges brought against Bill Browder. Photograph: Sergei Savostyanov/TASS

Russia’s most prominent opposition politician, Alexei Navalny, said on Twitter on Monday: “Interpol will soon elect a new president and it can be a candidate from Russia. Our team has suffered from abuse of Interpol for political persecution by Russia. I don’t think that a president from Russia will help to reduce such violations.”

Browder was once a cheerleader for the Russian president, Vladimir Putin, and the Kremlin, until his investment fund Hermitage Capital ran into problems. He was barred from the country in 2005, and Magnitsky, whom he hired to investigate his business troubles, was arrested in 2008 and died in prison in 2009.

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Since then, Browder has become a tireless campaigner against the Putin regime, travelling the world and lobbying parliaments to adopt sanctions against members of Putin’s circle and other human rights abusers globally, known as the Magnitsky Act.

On Monday, he tweeted: “Putin’s ‘fever dream’ response to being caught poisoning the Skripals is accusing me of four murders, including poisoning of Sergei Magnitsky.”

Browder has touched a nerve in Moscow and repeatedly comes up in Russian complaints. After a summit between Putin and Donald Trump in Helsinki over the summer, Putin suggested that Robert Mueller’s team could take part in questioning of potential suspects in the Russia inquiry, as long as Russian prosecutors received reciprocal access to Americans suspected of crimes in Russia. Putin named Browder as a person of interest. Trump initially referred to Putin’s suggestion as “an incredible offer” but later dropped the plan.

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The new charges against Browder are the latest grotesque twist in a case that has seen, among other things, Magnitsky put on trial posthumously. Authorities claimed he had conspired with Browder to defraud the Russian state, rather than blown the whistle on a huge fraud case involving corrupt officials.

There is evidence that Magnitsky was denied medical treatment and beaten by prison officials before he died. Russian authorities have always denied any foul play in his death, despite a presidential commission finding evidence of gross rights violations.