British MPs have called on Swiss authorities to scrap plans to return millions of pounds to a group of Russians sanctioned for their links to the Putin regime.
The funds were part of a $230 million (£190 million) fraud exposed by Sergei Magnitsky, a lawyer who died in a Russian prison after being beaten and denied medical treatment.
The Swiss courts have ruled that $15.3 million (£12.7 million) should now be returned to three Russians implicated in the fraud, despite a senior investigator in the case being convicted of accepting bribes, after the authorities failed to establish proof of a criminal conspiracy.
MPs are calling for James Cleverly, the Foreign Secretary, to not only raise the matter with his Swiss counterpart, but also consider whether to end co-operation with the Swiss authorities.
In a joint letter, Jonathan Djanogly, the Tory MP for Huntingdon, and Dame Margaret Hodge, the Labour MP for Barking, state: "The UK has made every effort to lead the way on holding Russia to account for Putin's invasion through [sanctions] and providing support for Ukraine.
"It seems unreasonable that our supposed ally would return clearly ill-gotten gains to these Kremlin-linked perpetrators. It also raises the serious question of how much more illicit Russian funds are available to Putin's officials in Switzerland."
The Swiss courts froze $19.6 million (£16.2 million) in 2011 as part of an investigation into alleged money laundering by Vladlen Stepanov, the husband of a Russian tax official; Dmitry Klyuev, the owner of a Russian bank and head of an organised crime group; and Denis Katsyv, the son of a senior Russian official.
The three were investigated for allegedly receiving funds as part of a plot by Russian interior ministry officials to defraud the Hermitage Capital investment fund of $230 million tax through bogus tax rebates.
When Magnitsky, the fund's lawyer and tax adviser, exposed the plot he was arrested and tortured, dying without trial in Nov 2009.
Mr Stepanov and Mr Klyuev were later sanctioned by the UK Government under anti-corruption laws named after Magnitsky. The pair have also been sanctioned by the US, Canada and Australia under similar laws.
Mr Katsyv paid $6 million (£5 million) to the US Department of Justice to settle its investigation into the Hermitage Capital case.
However, the Swiss Federal Criminal Court ruled last year that there was no proof of a "criminal organisation" running the tax rebate scam, paving the way for funds traced to Swiss bank accounts to be unfrozen and returned.
The three Russians have not been found by the Swiss courts to be responsible for bribes or money laundering.
In their letter to the Foreign Secretary, Mr Djanogly and Dame Margaret said: "During the Swiss investigation, the most senior investigator accepted bribes from his Russian counterparts to sabotage the case.
"Although he was fired and later convicted, Swiss law enforcement ruled last year that $15.3 million could be given back to the Russian individuals."
They added: "We ask you to immediately make representations to Ignazio Cassis, the Swiss foreign minister, to demand reassurances that these funds will not be returned to Russia."