The Lausanne-based Federal Tribunal said in its legal explanation that cooperation with Russian prosecutors "must be refused" because according to human rights observers the proceedings in Russia were politically motivated.

"The political and discriminatory character of the procedure in Russia is underlined by the infringement of human rights and of the right to defence which have apparently been committed throughout the procedure," the court said.

In issuing its ruling the court upheld the appeals lodged by six plaintiffs, two of which are thought to be former Yukos chief executive Mikhail Khodorkovsky and his associate Platon Lebedev, who are both serving prison sentences for fraud and tax evasion related to the running of Yukos.

Yukos itself has been effectively run into the ground after the Russian tax authorities imposed back taxes of almost $30 billion. The company was declared bankrupt in August 2006 and its main assets have been sold off to help pay these debts. Many questions have been raised about the way in which the Russian government handled the Yukos affair, and it is widely believed that both the jailing of Khodorkovsky and Lebedev, and the breaking up of Yukos were politically motivated and directed by the Kremlin; Khodorkovsky was a notable political opponent of President Vladimir Putin.

In January 2006, the Federal Tribunal rejected a similar request for judicial assistance from Russia relating to the handover of documents in relation to the Yukos case, saying that it could "not agree to collaborate with what looks more like a never-ending search for evidence." The judges also noted at the time the Council of Europe's reservations over the trials of Khodorkovsky and Lebedev.

However, Swiss prosecutors have released the estimated US$200 million of the remaining frozen assets related to Yukos held in Swiss bank accounts. This is all that remained of the US$5 billion originally frozen by the Swiss in 2004 - the largest amount of money ever frozen by Swiss authorities.

The Swiss authorities have said little about the latest decision to the media. Speaking to Reuters, Maria Schnebli, a federal prosecutor in the Swiss attorney general's office, confirmed that the freeze had been lifted on CHF200m to CHF300m held in Swiss accounts, but refused to elaborate, telling the news services that "that is the end of it." Meanwhile, the Russians are said to be reviewing the case before deciding on their next course of action.