RELATIONS between Russia and the UK have plunged further into the deep freeze after a prominent British Kremlin critic was arrested in Spain.

Financier Bill Browder was held in Madrid on Wednesday apparently under a Russian arrest warrant over allegations of fraud.

The self-professed “number one enemy” of Vladimir Putin was released around two hours after he was taken by officers to a station in the Spanish capital. He had travelled to Madrid to speak to a prosecutor who is gathering evidence about the Russian mafia.

Conservative MP Sir Peter Bottomley called his detention “absurd”, while Foreign Affairs Committee member Bob Seely said Moscow was “upping the ante” in going after its critics abroad.

Mr Browder, a US-born British national, claimed this was the sixth time Russia had “abused Interpol” in pursuing him.

He has been a thorn in the side of the Russian authorities for more than a decade after he tried to expose the “looting” of his investments in the country by corrupt officials.

In 2009, Sergei Magnitsky, a lawyer he hired to investigate what had happened to his money, died in Russian custody. Following his death, anti-corruption laws were introduced in a raft of countries including the United States and Canada. The UK is also staging a crackdown on Russian criminal money dubbed the McMafia laws.

Foreign Secretary Boris Johnson said on Twitter after Mr Browder’s release: “Just spoken to Bill Browder – very glad that he has now been released. Moscow should concentrate on bringing those responsible for the murder of #Magnitsky to justice.”

Detailing the events on social media, Mr Browder wrote: “Urgent: Just was arrested by Spanish police in Madrid on a Russian Interpol arrest warrant. Going to the police station right now.”

He said after his release: “Good news. Spanish National Police just released me after Interpol General Secretary in Lyon advised them not to honor the new Russian Interpol Red Notice. This is the 6th time that Russia has abused Interpol in my case.”

Sir Peter added: “It is absurd that a person with British nationality who has been exposing the brutal and fatal treatment of Sergei Magnitsky should be arrested at the request of the Russians.”