When did you last see these paintings?

The three priceless works of art on the right were snatched in a raid on a gallery in Rome, it was revealed yesterday. They are the latest additions to a hoard of treasures, stolen to order and never to be seen again...

TWO VAN GOGHS and a painting by Cezanne have been stolen from a gallery in Rome in the latest in a series of robberies which illustrates how art theft has become one of the most lucrative global criminal activities.

The National Gallery of Modern Art's skeleton night staff of three were left bound, gagged and unable to call the police who arrived on the scene hours after the robbers had made their getaway on Tuesday night with the three priceless paintings.

In the first armed robbery in an Italian museum since 1992, wardens were forced at gun-point to deactivate the gallery's alarm system as the three thieves removed Le Jardinier and L'Arlesienne by Van Gogh, and Cezanne's Le Cabanon de Jourdan. These masterpieces will be added to the Art Loss Register's list of works of stolen art - currently worth more than pounds 1bn.

"These works are so valuable, and so well-known, that they cannot be sold; at least, not on the normal market," said General Roberto Conforti, the head of the police art-theft squad.

So, if these paintings are too celebrated to be sold on the open market, why steal them? According to Charles Hill, former head of Scotland Yard's arts and antiques squad and now a member of Nordstern, a leading art insurance group, the answer lies in the psychology of the thief. "They steal to fashion, not to order," he said. "It's a cachet crime committed by greedy thieves looking for a high-profile crime to make their name. It's a really dumb crime stealing a masterpiece - because you can't sell them on."

Mr Hill cited as an example Peter Scott, once known as "the human fly" as a result of his spectacular career as a cat burglar, who was jailed on Tuesday for his part in a plot to sell a stolen pounds 750,000 Picasso painting. The officer who led the police operation said he believed Scott "revelled in infamy".

In such cases, it is not uncommon for a ransom to be offered after the theft. Yesterday afternoon, police in Rome were investigating an anonymous phone call to an Italian news agency, in which a man said that a ransom demand for the return of the paintings would be made soon. He said this would include "political demands". Once such a call has been made, the chances of recovering the work of art improve.

Cultural heritage minister Walter Veltroni promised "utmost commitment" to recovering the stolen works, and pointed out that the use of weapons in art thefts in Italy was almost unknown. "This is a qualitative leap," he said. Mr Veltroni defended Italy's recent record on protecting its vast cultural heritage, reporting that art thefts were down 40 per cent in the first months of 1998, and that around 50 per cent of all stolen art works are recovered.

Gallery spokeswoman Elena di Majo had a different theory about the motive for theft, pointing to the fact that the paintings were stolen from a room containing works by major 19th-century artists, including Degas, Monet, Courbet and Klimt. "The robbers left behind a lot of great paintings worth just as much, if not more than the ones that were stolen," she said. "It looks very much like they were acting on commission."

Experts differ in their opinions of whether such paintings are destined for a private collector after being stolen to order. The popular image of Ian Fleming's Dr No, who has art stolen to order and preserved for his private viewing, is dismissed as myth by Mr Hill. But others, such as Colin Norvelle-Read of Trace magazine, which publicises stolen art and antique treasures, maintain that Dr No characters who revel in their secret hoards of stolen objets d'art do exist.

"Some collectors are quite obsessional about a particular piece and go to any lengths to actually get hold of it," said Mr Norvelle-Read. "It will just be that when they walk into their secret room with their collection of Lowrys or whatever, they get a lot of pleasure out of looking at the collection."

Thefts of instantly recognisable works such as the Van Goghs and Cezanne stolen on Tuesday night only occur about three times a year worldwide. While such paintings tend to remain hidden, the majority of art booty is regarded as international currency, employed as collateral in underworld deals or handed over to banks unaware of the paintings' provenance in return for loans.

"Commission theft does happen," said Caroline Wakeford, operations manager at the Art Loss Register, "but there's a much more sinister reason. The art is usually used as collateral in crime linked with drugs and arms dealing. It's like a loan note." Caravaggio's Adoration, stolen in 1969 and unrecovered, is said to have passed between mafia bosses as collateral.

Among the missing works or art recorded on the Art Loss Register in London are 349 Picassos, 250 works by Marc Chagall and 175 by Salvador Dali. The register keeps an eye on what is put on sale at auction houses and checks them against items reported as missing.

Missing

The Concert by Jan Vermeer - stolen in 1990 from a museum in Massachusetts: priceless

Missing

The White Duck by Jean-Baptiste Oudry - stolen 1992 from an estate in Norfolk: value pounds 5m:

Missing

Storm on the Sea of Galilee by Rembrandt - stolen at same time as The Concert: priceless

Missing

Sibylle von Cleve by Lucas Cranach the Younger, stolen from a castle in Baden, Germany, 1995

Missing

Still Life by Georges Braque, a contemporary of Picasso, stolen from a Stockholm museum

Missing

Shade and Darkness - Evening of the Deluge, by Turner, stolen from Frankfurt 1994: pounds 10m

Missing

Portrait of a Lady by Gustav Klimt - stolen from Piacenza in Italy, 1997

Missing

Turner's Light and colour - the Morning After The Deluge, also stolen 1994 from Frankfurt: pounds 10m

Missing

Adoration by Caravaggio - stolen in 1969, stolen from Palermo's San Lorenzo Oratory: pounds 20m

Missing

Nebelschwaden by Caspar David Friedrich - taken from Frankfurt museum, 1994: pounds 1m