Dutch coffee shops, long considered as synonymous with the Netherlands as tulips or attacking football, face a new challenge from today when a ban on smoking tobacco in restaurants and cafes comes into effect.

The owners claim the law, which will allow customers to light up potent tobacco-free pure cannabis joints but ban milder spliffs in which tobacco is mixed with cannabis, threatens to put hundreds of them out of business.

"It's a bit like saying to someone you can go into a cafe and you can buy a beer, but you can't drink it there - you'll have to stick to whisky, rum and vodka," said Paul Wilhelm, owner of De Tweede Kamer, a popular Amsterdam coffee shop.

As most patrons prefer milder joints in which cannabis is mixed with tobacco, and only 18% favour much stronger, pure cannabis spliffs, the fear is that the days of the coffee shops could be numbered.

The catering industry said 1,600 coffee shops across the country were up for sale because their owners were convinced their businesses were doomed.

Wilhelm, who has run his cafe since 1985, said the law was in danger of "tearing the heart out" of Amsterdam's social life. "The focus of the De Tweede Kamer has always been social contact," he added. "They'll destroy that with what I see as a ridiculous law."

Mark Jacobsen, chairman of the BCD, a nationwide association of coffee shop owners, said proper implementation of the law would require inspectors to check each cannabis joint for tobacco content.

"It's absurd. In other countries they look to see whether you have marijuana in your cigarette, here they'll look to see if you've got cigarette in your marijuana."

The Food and Consumer Product Safety Authority, which is responsible for enforcing the ban, said it had trained around 200 inspectors. "They can tell the difference between a mix or a pure joint from its smell and appearance," said a spokesman.

Some coffee shop owners have sought alternative ways to deal with the conundrum and allow their clientele to still enjoy a tobacco-free cannabis spliff without it being too strong.

One inventor, who calls himself Evert, is doing a roaring trade in £400 vapourisers - in which hashish or cannabis is heated to 180C and the resulting steam is directed into a balloon from which smokers inhale. The vapouriser is claimed to be the mildest way to consume pure cannabis.

While some cafes have said they will simply lay on more pure cannabis brownies or "space cakes", others have built smoking chambers within their premises which are off-limits to staff. Some are also providing alternatives to tobacco, such as the herb coltsfoot.

Martial von Beenkom, owner of Boerejongens (Young Farmers) on the outskirts of Amsterdam, has turned his cafe into a smoke-free tea salon where he will sell cannabis on a takeaway basis. "Most people come here to buy their grass which they then smoke elsewhere," he said.

The health minister, Ab Klink, said the law would stay, arguing that as well as helping to improve people's health it might help to stamp out idleness. "Consumers who spend the whole day hanging out in coffee shops will find other things to do," he said.

Back story

The tradition of the Dutch coffee shop goes back to the 1970s, following the pragmatic policy of non-enforcement of laws on the consumption and possession of cannabis that was introduced in 1976.

Under the policy licensed coffee shops are allowed to sell five grams of cannabis to customers over 18.

Last year, the country's 750 coffee shops, half of which are in Amsterdam, were forced to choose between serving alcohol or cannabis. Most chose cannabis.