with Russia's state-oil companies because of his alleged friendship with Putin.

In December 2007, the Russian political scientist Stanislav Belkovsky told the Guardian Putin had secretly amassed a $40bn fortune. Putin was the beneficial owner of "75% of Gunvor", he claimed, adding that Putin's ownership was concealed through a "non-transparent network of offshore companies". Putin denied the claim three months later.

In a letter to the Guardian in December 2007, Gunvor's chief executive, Törbjorn Törnqvist, said Putin "was not a beneficiary of its activities". However, he confirmed for the first time the men had been friends since the 1990s. "Timchenko did indeed know President Putin in the days before the latter became famous," Törnqvist wrote. "However, suggestions that they share a KGB heritage or have been in business together arecompletely wide of the mark."

Last year the Guardian sent Timchenko a list of questions asking him to elaborate on the precise nature of his relationship with Putin, and seeking more details as to how and where they first met. Timchenko declined to answer. The billionaire has so far given only one media interview, to the Wall Street Journal last year.

It is not clear why Timchenko sued the Economist, rather than write a letter of complaint. The Economist article contains two controversial paragraphs, and appears to repeat claims made by the Financial Times last May.

In a letter last year to the FT, Timchenko said he had sponsored a judo club in St Petersburg that had Putin as its honorary president. He denied this meant he was "close" to the