When did you last see your Turner?

Last weekend two armed men, calling themselves Bert and Tony, burst into York City Gallery, and bound and gagged four staff before making off with pounds 1m worth of paintings. The curator, Richard Green, said the theft of 20 works of art by men wielding a pistol and sawn-off shotgun, was the "darkest day" in the gallery's 100-year history. The art world fretted over the implications of the theft, but opinion was divided. Were the thieves professionals, or a couple of hicks who would not know a Constable from a Warhol?

Early reports claimed that police believed the raiders were "stealing to order" - that they knew what they were looking for and may have had links with drug gangs. Ports and airports were alerted to stop the paintings, including a Turner watercolour (pictured) and two works by the Italian master Bartolommeo, being spirited abroad.

North Yorkshire police, however, are sceptical about the thieves' organised crime links. Charles Hill, the former head of the Metropolitan Police's art and antiques squad - and now a risk manager with the art insurers Nordstern - points out that the criminals stole pounds 20 from one of the gallery staff after the getaway car ran out of petrol. "These are probably two dumbos from Doncaster," he says. "The most stupid thing a criminal can do is to steal a famous painting. They are almost impossible to sell on."

Old masters occasionally turn up in bin bags outside police stations, or are ransomed back to galleries by thieves after failing to flog them. Mr Hill believes the Turner is probably lying "on the top of a wardrobe somewhere in York or Doncaster".

Mark Dalrymple, a fine art loss adjuster and chairman of the Council for the Prevention of Art Theft, says the York paintings are likely by now to be acting as collateral in criminal deals. An estimated pounds 500m worth of paintings, fine art and antiques is stolen every year in Britain, and pounds 3bn worth is stolen world-wide. It may be near impossible to sell a stolen masterpiece, but the big names keep going walkabout.

Among those on the international missing list are 355 Picassos, 271 Miros and 250 Chagalls. An investigation into organised crime in Italy revealed that Caravaggio's Adoration, stolen in 1969, was passed between a series of Mafia bosses before being destroyed. According to Mr Dalrymple, something similar is happening in Britain.

In his fight to recover paintings, Mr Dalrymple, working closely with the police, offers rewards for information and cultivates connections in the criminal underworld. Among a host of missing treasures, he is currently pursuing two Turners (worth pounds 24m) stolen in 1994 in Frankfurt while on loan from London's Tate Gallery, and a Titian (worth pounds 6m) stolen in 1995 from the Marquis of Bath's Longleat estate. He says he knows from criminal connections that one of the missing works of art - he will not say which - has figured in a series of criminal deals.

That is not to say that those who do the stealing know anything about art. He recalls a recent break-in at a private home when a pounds 750,000 Kandinsky was passed over in favour of a pounds 10,000 painting sharing the same wall.

Stolen art can change hands quickly. It is sold for cash - at a fraction of its open-market value - then swapped for drugs or used as collateral for loans. Despite the criminal world's growing affection for works of art, however, armed robberies on galleries are still so rare that the memory lingers. The most famous, an attack on Sir John Soane's gallery in London in the Eighties, was a bloody affair. Police were waiting for the gang and, in the gunfight which ensued, one robber was killed. Bullet holes still decorate the gallery entrance.

More recently - in 1997 - Russell Grant-McVicar, son of the famous former bank robber John McVicar, held up a London gallery at gunpoint before running off with Picasso's Tete de Femme, worth pounds 650,000. Perhaps any operation in which the getaway car is a taxi is bound to fail. Just 10 days later, Peter Scott, the celebrated "gentleman thief", who famously boasted that he once stole Sophia Loren's knickers as well as her jewels, was arrested trying to sell the painting on.

Mr Dalrymple says the York raid - just three weeks after an armed attack on an Oxford Street gallery - "is a sign that the gangs are getting bolder". Pictures of the York paintings will soon be circulated by Trace, an art industry body which works with the police to recover art and antiques. They will join 100,000 other treasures on the database.

Overall recovery rates, says Mr Dalrymple, remain "pitifully" low. Katrina Burroughs of Trace says art is seen as a soft target. With police resources scarce, the industry tries to help itself. In March it will launch a new "due diligence" code, backed by government, which will place more onus on dealers to establish that the works they buy are not stolen.

Of course, the successful art thief does not steal an old master. It is the "second division" works - paintings, clocks, furniture, silverware, not well known but very valuable - which bring profits with least risk. They are housed in poorly secured municipal galleries, country houses and private homes all over Britain. The elderly, often physically or mentally vulnerable and with no idea how valuable their possessions have become, are major prey.

Robert Heal, 94, remembers "the woman from social services" who came to call last year, to check he was being looked after. Mr Heal was physically fine but, widowed just the year before, chronically lonely.

"She was very nice," says the retired architect. "I told her all of my contemporaries had died off and it was hard to make new friends."

The social worker came with an assistant. Perhaps callous crime requires a dark sense of humour but she told Mr Heal the assistant would handle any heavy furniture which needed moving. During a later visit she drugged Mr Heal with sleeping pills, and her assistant moved selected pieces into a waiting van. "When I woke up I went to check the time," Mr Heal says. "But the grandfather clock was gone." So was a Mary and William cabinet, other antique furniture and a valuable early-19th-century painting.

Charlene Davis, 41, and her boyfriend Duncan Cousins, 34, an antiques dealer, were jailed for seven years. Police insisted at first that the theft at Mr Heal's was an isolated case, until an officer in another area spotted similarities with local crimes. Investigations led to the couple being convicted of thefts in 400 houses, all over Britain, involving pounds 6m worth of art and antiques.

This case shows how easily stolen works can still pass through respectable art circles. Mr Heal's stolen picture passed through the hands of four unsuspecting art dealers, including David Collins, the paintings expert from the Antiques Roadshow, before being traced to South Carolina.

Last week's events must have left municipal art galleries with an acute sense of vulnerability. They have probably never considered themselves targets for gun-toting gangs. Many galleries - like York City - do not even have their works insured. In accordance with council policy they take a "calculated risk".

"We just never envisaged something like this," says Richard Green, the gallery's curator for more than 20 years. "One fears a new chapter (in art thefts) may be opening up."