What is the Magnitsky Act and how does it relate to a British anti-corruption campaigner's arrest in Spain?

The detention and release of anti-corruption campaigner Bill Browder in Spain has again cast an unfavourable spotlight on Russia.

Police in Madrid picked up Mr Browder on a warrant issued by Russia before setting him free on the advice of Interpol's secretary general Jurgen Stock in Lyon, France.

The former investment fund boss, born in Chicago but of British nationality, believes he was detained by order of the Kremlin on false tax evasion accusations as revenge for his work with US Congress to pass the Magnitsky Act.

But what is that law and how does it relate to President Vladimir Putin?

The Russia and Modova Jackson-Vanik Repeal and Sergei Magnistsky Rule of Law Accountability Act of 2012 takes its shortened name from Russian lawyer Sergei Magnitsky, who uncovered a tangled web of tax fraud in 2008 involving 23 companies and $230m linked to high-ranking officials in the Russian government.

Mr Magnitsky was investigated, arrested and died in prison in 2009 in mysterious circumstances.

Mr Browder – whom Mr Magnitsky worked for while the former was head of Hermitage Capital Management, at one time the largest foreign portfolio investor in Russia – believes his friend died as a result of torture and has become an outspoken critic of the Putin administration ever since.

The legislation was signed into law by President Barack Obama in December 2012 and bars Russian officials suspected of human rights abuses from entering the US, allows for their assets held in US banks to be frozen and prohibits future trading with American financial houses. Initially, 18 Russian businessmen were hit by the ban, a total now risen to 44 following an expansion of the act in 2016.

Moscow retaliated by blocking the adoption of Russian children by US parents, posthumously finding Sergei Magnitsky guilty of the crimes against the state and imposing sanctions on Mr Browder.

Vladimir Putin's government thereafter continued to pursue Mr Browder, appealing to Interpol to have him arrested on three separate occasions between 2012 and 2015 until the Council of Europe intervened to accuse it of "abusing" the system.

The Magnitsky Act returned to the news again in 2017 when it emerged that lawyer Natalia Veselnitskaya – who controversially met with President Donald Trump's son Donald Trump Jr and son-in-law Jared Kushner at Trump Tower in New York on 9 June 2016 to discuss information she claimed she had to the detriment of rival presidential candidate Hillary Clinton – had been long involved in work to see the act repealed.

Ms Veselnitskaya also represents wealthy businessman Denis Katsyv, fined by the US Justice Department in 2017 over money laundering allegations, who is also committed to seeing the Magnistsky Act overturned.