When a Rubens is not a Rubens

There is a story that Picasso accompanied a colleague to an auction to reveal which of the paintings attributed to him were forgeries. As example after example of the great artist's work was held up, Picasso muttered "fake" to each. At the end, his bemused colleague exclaimed "my God - how can so many of your works have been faked?" to which Picasso answered: "All art is fake."

The point of the story may be more oblique, but recent revelations about the extent of imitation and misattribution in the art world leave the viewer wondering which works are what they say they are and how it is possible to tell.

Last week alone doubts were cast on two major works; the National Gallery agreed to scientific testing of one of its prized Rubenses - Samson and Delilah - after a group of artists claimed it was a copy; while the portrait of Lady Jane Grey at the National Portrait Gallery has been discreetly re-identified as being of Catherine Parr, the last wife of Henry VIII.

And on a more sinister front, it emerged this month that Scotland Yard is investigating a huge modern art fraud that stretches back six years and casts doubts on the provenance of works by, among others, Ben Nicholson and Alberto Giacometti.

These are not isolated examples; there are long-standing doubts about many works currently on show. There are rumours that the Royal Academy houses a painting by the forger Tom Keating, while the autobiography of faker Eric Hebborn revealed a list of Hebborns in galleries across the world.

Virtually no major institution is exempt; the Courtauld Institute confirmed that its 15th-century Madonna and Child altarpiece was really the work of an early 20th-century faker I F Joni. Its apparent antiquity was belied by the discovery of the modern nails which, hidden by gesso, held the piece together.

And in 1994 the National Gallery's "Young Michelango" exhibition, which focused on two unfinished pictures by the artist hit the headlines after an expert claimed the larger of the two, The Entombment, was by a lesser contemporary.

Each "revelation" prompts debate and hand-wringing within the art establishment, but the truth is that the process by which works are attributed is liable to change with time. The Rubens mentioned above had borne a label for 237 years advising "do not attribute this painting to Rubens". And the Rembrandt Research Project, which spent 25 years deciding which of 630 works attributed to the master were fake, has since seen many of its pronouncements overturned.

In a market apparently suffused with works that are not what they seem, and in an age where the quality of reproduction makes it increasingly hard to tell, does it all matter?

According to Michael Newman, head of theoretical studies and art history at the Slade School of Fine Art, until the early Renaissance a painting was only worth the cost of the materials and pigments used to produce it. "As the artist became more important, the artist as author of their work became a phenomenon of early Renaissance," he says. Even then it was common for artists to "contract out" parts of a painting to other artists. Today's art historian, he says, must hunt for the tiniest details that will determine true authorship.

But does the fact that a work might not be an original alter the casual viewer's enjoyment of it? Auction houses now house sales of reproductions of masterpieces, while the popularity of the True Fakes Gallery in New York is testament that fakes themselves can now achieve values almost comparable with the originals. Bonhams has sold a work by Tom Keating for pounds 27,500.

Meanwhile, new methods of art mean the concept of originality may itself prove problematic. With conceptual art - the medium of the 1990s - only the concept can be genuine. Damien Hirst, for example, did not catch his shark, nor did he gut it. Does the fact that all this work was contracted out mean that it is still all his work? And in the future, how will we be able to tell?

Finally: a confession. The Picasso story at the start was a fake.