They used to adorn everything from park benches to cathedrals, but growing numbers of azulejos -- the painted ceramic tileworks that symbolise Portuguese culture -- are being lost to a booming trade in stolen art.

Each year thousands, even tens of thousands, of azulejos are taken. From individual tiles snatched by tourists from the walls of the capital Lisbon to large and highly valuable figurative works depicting 18th-century religious scenes, the theft of azulejos has grown exponentially in recent decades.

Some of the artworks are estimated to be worth tens of thousands of euros.

"Since 1990, we have seen an explosion of thefts with peaks in 2001-2 and 2006 and with more than 10,000 azulejos reported stolen every year," said Leonor Sa, curator of the Portuguese Police museum.

"But those reported stolen only represent a tiny percentage of those taken. The loss to the national heritage is colossal," he said.

In a bid to stop the drain, Sa came up with the idea SOS Azulejos, a recovery project run by the national police. Launched last February, it notably maintains a website, www.sos.azulejos.com, which carries when available photos of "missing" azulejos.

Over the years many of the stolen items have been resold to genuine art dealers.

"Three weeks after the opening of the site I discovered that I had in my possession a work stolen from a palace in Lisbon in 2001," said Manuel Leitao, a reputed antiques dealer specialising in Portuguese azulejos.

The blue and white artwork of a sailing ship was made up of some 70 painted squares of 14 centimetres (5.5 inches) each.

"It was sold to me by a man who said he had inherited it from his father," said Leitao, adding that the thefts now appeared to be "unstoppable" whatever the level of security in place.

Police investigator Joao de Oliveira says the Portuguese are so accustomed to living their lives surrounded by azulejos that they have come to take their existence for granted.

"There are so many of them -- the Portuguese are born in hospitals decorated with azulejos, live surrounded by them, marry in churches covered by them and are even sometimes buried in cemeteries with azulejos -- that they do not pay them any attention to them," he said.

But stealing large 18th-century works -- some of which have hundreds of individual tiles -- is not like stealing a car, according to Oliveira who runs a police squad specialising in stolen art works.

"It requires resources, contacts and qualified staff. And this targeting of azulejos leads me to think that it is well organised.

"When thousands of azulejos are stolen from a locked church where the tileworks completely cover the walls up to six or seven metres (19 to 23 feet) high, we know full well that this is not some smalltime thief," he added.

Drug addicts looking for a dozen tiles to sell to a junk shop to fund their next fix are thought to be responsible for only a small number of thefts.

"The big pieces are not stolen by them," said Oliveira who is "convinced of the existence of a parallel market" for the tileworks.

Authorities say one of the problems they face is that any seller who is registered in Singapore, for example, is beyond their reach.

"You cannot do anything. We know who it is but that is not enough. You have to prove it," added Oliveira.

With the number of thefts continuing to rise, the police officer is convinced that improving awareness is the key to combating the illegal trade.

"Nothing will ever happen without more awareness among the authorities, private owners and also the public, of the richness of their artistic heritage and the need to preserve it," he said.

For art historian Jose Meco, azulejo art in Portugal is not only central to the culture but also unique.

"There are more azulejos in Lisbon than in the rest of the world. But what makes them important in our culture is their incredibly extensive use," he said.