California-based FBI fraud investigators have been called in to assess the legality of a recent sale by US online auction house eBay.

According to US reports today, the FBI are said to be probing the online sale of what was rumoured to be a valuable work of modern art by Richard Deibenkorn, an American abstract expressionist painter who died in 1993.

The canvas was put up for sale by Kenneth A Walton, a 32-year-old Sacramento lawyer.

The FBI investigation may centre on the possibility that a 'ring' of bogus bidders was organised to artificially inflate the price of the apparent masterpiece.

Walton denied any knowledge of an investigation, saying the first he knew about the controversy was through the media.

The affair began on 28 April when Walton used an internet alias - Golfpoorly - to put the picture up for sale on eBay without attributing it to Deibenkorn. He explained that he bought it at a garage sale in Berkeley, California.

A photograph, which went up on the eBay website, was said to show a resemblance to the work of Deibenkorn, with a visible signature reading 'DB52'.

Bidding was slow, but rumours of the possible provenance of the painting drove bids up, and after two weeks a mysterious Dutch bidder put in a surprise successful bid for $135,805 without any further identification of the picture.

The bid is believed to be the second highest ever made online for a work of art, a claim that made the headlines and thus attracted the attention of the FBI.

Suspicions were apparently aroused at eBay as well, when it was discovered that Walton himself had put in a bid for his own painting of $4500, a move which is against the company's rules.

When challenged, he told eBay staff that he was "bidding for a friend". He insisted his bid had no effect on ramping up the price of the canvas.

Staff at eBay are said to be assisting the FBI investigators in their quest to unravel the affair.