One in 10 Scottish 15-year-olds regularly uses cannabis, an official survey of teenagers nationwide reveals today.

More than a quarter have experimented with the drug and thousands are "heavy users", according to the groundbreaking study which is supported by the World Health Organisation.

Professor Candace Currie, principal investigator on the project, said so many young people were trying the drug it might even be called "normal behaviour".

The findings come amid controversy about the status of the drug, which is now available in stronger forms and has been linked to mental health problems.

Prime Minister Gordon Brown is pushing to increase the punishment for cannabis possession even though Westminster experts have advised it has not led to increases in schizophrenia and should remain a class C drug.

The Scottish study involved more than 6000 pupils and examined the lifestyle of young people aged 11, 13 and 15 in March 2006.

It is part of the Health Behaviour in School-Aged Children programme, which has been tracking the choices teenagers make since 1990.

There is some evidence adolescents have taken on board health messages during that time. The proportion eating sweets, crisps and chips daily has decreased significantly while use of contraception has increased.

The number of 15-year-olds using cannabis heavily has also dropped, from 6% to 3% - although, with more than 60,000 S4 pupils in Scotland, this still suggests in excess of 18,000 frequently take the drug. But the proportion of regular users - those who took cannabis up to 39-times in the previous year - has remained constant.

Alistair Ramsay, the former director of Scotland Against Drugs who now runs a company which advises on the problem, said the findings were worrying.

"There were a number of recent studies from New Zealand where they looked at the implications of cannabis on the brains of young people," he said. "They found there was long-term damage done to the brains of young people if they started too young."

He added there was always a risk teenagers who took cannabis would progress to other, more dangerous substances.

Mr Ramsay also called for more drugs education in schools saying that, since Scotland Against Drugs was shut down, there had not been a proper prevention programme.

However, Professor Currie said whether or not the level of drug-taking was worrying depended on perspective.

She said: "Probably to the young person it is not worrying at all because it is something a lot of their peers are doing. To a parent, it is probably fairly worrying because they think any contact with the drug is a worrying thing."

The survey questionnaires are filled in under exam conditions and Professor Currie said she was confident responses were generally honest.

Smoking and drunkenness has increased among 15-year-old girls since 1990 and levels of fruit and vegetable consumption remain low, but in many other areas the survey found teenagers are making healthier choices.

A Scottish Government spokesperson said: "Research has shown that substance misuse among school age children is declining. However, there is no room for complacency as usage is still too high.

"Our goal is for better substance misuse education to help young people make the right choices. We are aiming for all young people to be aware of the implications of the use and abuse of substances and to aim for a life free from substance misuse."