The debate over drug laws will be reignited next month when one of the world’s leading experts is to argue that Scotland should legalise cannabis to dramatically cut the country’s soaring heroin addiction rates.

Ethan Nadelmann will warn that the current UK drug policy is “damaging” and that the Scots should embrace the Dutch “coffee shop” model, under which cannabis is legally sold over the counter in licensed outlets.

Nadelmann, executive director of the US-based Drug Policy Alliance, and who is regarded as one of the most respected advocates of drug reform, will make the arguments at the prestigious Edinburgh Lectures series next month. Previous speakers at the event have included former Russian president Mikhail Gorbachev and the scientist Professor Stephen Hawking.

As well as urging the widespread introduction of so-called “brown cafes” in Scottish towns and cities, Nadelmann will also say the government should adopt a policy of “controlled legalisation” for hallucinogens, such as LSD, and an extension to heroin prescribing.

He said: “If you define legalisation as the way we treat alcohol, making it available over the counter to anybody over the age of 18, then my view is that cannabis should be treated in the same way.

“The Dutch coffee shop approach provides a very good model. Cannabis is just as easy to get in Scotland as in the United States. Anyone who wants to obtain it, can obtain it. So why keep the entire thing underground? Why not find a way of bringing it above ground and regulating it?”

In the Netherlands, the sale of small quantities of cannabis for personal use in cafes is permitted. The outlets allow patrons to openly smoke joints without fear of arrest.

With more than 51,500 estimated heroin-users in Scotland and record numbers of pregnant women addicted to class-A drugs, Nadelmann said he believed cannabis legalisation would reduce the number of young Scots indulging in hard drug use.

“When the Dutch adopted the coffee shop system, they found that the percentage of young people using cannabis who went on to use harder drugs declined,” he said.

Under this approach, he claimed, the government would eliminate the prospect of a criminal drug dealer turning a cannabis smoker onto harder drugs. “Most people who use cannabis don’t go on to use other drugs. But obviously if you have the same dealer selling all of these drugs together it increases the chance that people will use them,” he said.

“If you have a regulated system where people are held responsible and will be closed down if they sell any white powder drugs, you can effectively segregate the market.”

While acknowledging recent research linking cannabis to mental illness, Nadelmann added that this was “a reason to make cannabis more regulated”.

The controversial talk comes at a critical point in the UK debate on drugs. Next month, the government’s Advisory Council on the Misuse of Drugs is to report to Home Secretary Charles Clarke on whether cannabis should be restored to Class B status. This comes four years after then Home Secretary David Blunkett downgraded cannabis to a Class C drug.

Last night, Nadelmann’s comments prompted anger from drug researchers and politicians. Others, however, said he did not go far enough.

Professor Neil McKeganey, director of the Scottish Centre for Drug Misuse Research, said legalising cannabis was “enormously risky”.

He said: “We have a picture of widespread underage drinking and underage smoking in Scotland, and we could see a similar pattern with drugs if these substances were to be legalised. There is no reason on earth if they were legalised that there would be a drop in use .”

Margaret Mitchell, the Conservative deputy justice spokeswoman, described Nadelmann’s comments as “unbelievable”. She said: “If that’s his message, then he’s the last person we want to have here. Zero tolerance is where we should be starting from .”

Kenny MacAskill, the SNP justice spokesman, said: “Rather than making supply easier, I would argue that we need to reduce demand. We should invest in sports and arts, in giving folk, young and old, other outlets than the pub or smoking a spliff.”

Tom Wood, former deputy chief constable of Lothian and Borders Police and now chairman of the Action Team on Alcohol and Drugs in Edinburgh, which invited Nadelmann to the capital, said: “The idea to have him over here is to stimulate thought and give us food for thought. It’s healthy to listen to carefully considered views, even if you don’t necessarily agree with them.”

Kevin Williamson, the publisher and author whose Edinburgh cannabis cafe was closed down, said: “As long as cannabis is illegal young people will come into contact with heroin.”

Danny Kushlick, director of pro-legalisation group Transform, said: “No drug is made safer in the hands of criminals.”

A Home Office spokeswoman said there were “no plans” to legalise cannabis.

Ethan Nadelmann’s lecture, The Global War On Drugs, takes place on November 1 at 6.30pm in Edinburgh City Chambers.