The A73 south of Nijmegan was littered with bullet casings, and one man lay dead in his car with another sprawled wounded in the passenger seat.The survivor refused to talk to police, even though a hired assassin had pursued his vehicle shooting at it without hitting for several miles before finally catching up and riddling it with automatic fire.

Commuters were horrified, but the murder in September was wearily familiar to detectives who have dealt with 25 gangland-style killings in suburban southern Holland over the past three years.

As usual, there was a cannabis connection. The assassin was a hired Bulgarian and his two victims, men in their twenties, had been involved with one of the thousands of cannabis “nurseries” which flourish out of sight in the attics, sheds and spare rooms of small towns – using Dutch horticultural expertise honed from years of growing tomatoes and tulips.Billions of euros worth of cannabis is grown for export – much of it to Britain – in Holland’s modern cannabis industry, which has come a long way since the days of penniless hippies growing pot on Amsterdam houseboats and opening “coffee shops” where stoners could happily puff away in an atmosphere of dope haze, peace and love.Now there is so much money and violence involved that Holland’s police commissioner responsible for cannabis calls it a danger to Dutch society.Since he started his job a year ago Max Daniel has made it his mission to change Holland’s laid-back view of the drug, and as calls mount from politicians and citizens to shut “nuisance” coffee shops he believes that his message is getting through.Mr Daniel said: “For years this was seen as an innocent business and the tolerant Dutch approach was undoubtedly a successful form of harm reduction – it kept users away from hard drugs.

“But now there is so much money to be made that cannabis is sucking in organised crime gangs from abroad and corrupting legitimate businesspeople – especially lawyers, estate agents and bankers. Money laundering is a massive enterprise, and it is bringing together white-collar professionals and the kind of criminals who deal with heroin, prostitutes and people-smuggling.

“Cannabis is a threat to our democracy.”Mr Daniel said police noticed that the business was starting to change about 15 years ago when criminals realised there were bigger profits from growing cannabis in Holland than smuggling it from Morocco, but the violence has become much worse in the past few years.Dutch police believe that the underground cannabis growing cottage industry has now become one of their nation’s biggest earners of foreign currency, worth an estimated 2.7 billion euros (£2.3 billion) in total – about half as much as Holland’s legitimate horticultural business.The public perception has not kept up with the worsening criminality; most Dutch still regard cannabis as harmless, if not quite respectable. A nationwide poll in November found that 80 per cent of Dutch people opposed the closure of marijuana coffee shops.The nation’s 730 coffee shops, where customers can buy herbal cannabis or hashish without fear of arrest, attract tourists and pay more than 300 million euros in tax annually.An estimated 40 per cent of the cannabis grown in Holland is sold in them. Police believe some are fronts for organised crime, but the worst of the violence takes place in the cannabis-growing industry where strong-arm gangs prey on novices who think they can make easy money by setting up cannabis farms.Everything needed can be bought in a “grow shop” – seeds, nutrients, powerful lights and hydration systems. Police say some grow shops sell the addresses of novices to criminal gangs, who months later smash their way in and steal crops or cash.Cannabis growers can’t go to the law for protection, so they arm themselves, electrify doors to shock or electrocute, or buy large dogs for protection. In one case police discovered a trap for intruders, in the form of a pit filled with sharpened stakes dug beneath a doormat. Suburban Holland has never seen anything like it.

Public anger about tolerant drugs laws is mounting along the French and Belgian borders, where rows of coffee shops sell to thousands of drugs tourists every week. They are accused of making a nuisance in the placid and law-abiding small towns.

This month Amsterdam’s civic fathers decided to shut 43 of the city’s 228 coffee shops as they were close to schools, another sign of growing anxiety about the city’s laid-back drugs laws.So far coffee shop owners have been remarkably relaxed in the face of the growing campaign against them.