HEROIN, cocaine and ecstasy users should face police warnings instead of prison if found with small amounts of drugs for their personal use, the Scottish LibDems will argue this week.

The party will use its Spring conference to advocate decriminalising drug use - as opposed to drug dealing - in a fundamental reform of how addiction is dealt with by the authorities.

Leader Willie Rennie will ask delegates to make treating drug use a health and social issue rather than a criminal one a key manifesto pledge for May’s Holyrood election.

Writing in today’s Sunday Herald, he says: “We’re not winning the so-called war on drugs and we have to consider the alternatives. We do not believe vulnerable people struggling with addiction should be imprisoned simply for possessing drugs for personal use.”

Possession of a Class A drug like heroin is currently punishable by up to seven years in jail.

Neil McKeganey, director of the Centre for Substance Use Research in Glasgow, called the plan “naively optimistic” and said it would inevitably increase drug use.

Although high-level drugs policy is reserved to Westminster, the LibDems argue that day-to-day prosecution and sentencing guidelines can be fine-tuned within Scotland.

A policy document going before next weekend’s conference in Edinburgh will call for a change to the guidelines “so people caught in possession of small amounts of drugs for personal use are referred for treatment, education or civil penalties, ending the use of imprisonment which is costly and ineffective”.

The plan is to build on a new system of on-the-spot recorded warnings being used by Police Scotland for those caught with small amounts of cannabis.

Introduced last month, the warnings cover a range of low-level offences which were previously reported by police to prosecutors, but which rarely ended up in court. They are now being issued to people caught with small amounts of cannabis.

Rennie says warnings should be extended to all classes of drugs held in small amounts.

McKeganey said of the plan: “It’s exceptionally starry-eyed. The idea that you can simply differentiate between someone who’s using drugs and dealing drugs is a fantasy.

“There’s a merger of those two groups, with users getting money by selling drugs. It’s so detached from the reality of the problem.

“I think it’s likely there would be an increase in drug use in Scotland without the risk of prosecution. We should not be introducing any policy that would inevitably worsen a drug problem that is already one of the worst in Europe.”

However David Liddell, Chief Executive at the Scottish Drugs Forum, said: “Recent changes in the police practice in Scotland around possession of small quantities of cannabis have brought us more or less into line with England in terms of policing possession.

“There is an opportunity to build on this and consider similar moves for other substances. The key issue for us is with regard to reducing drug-related harm. We welcome a debate on how decriminalisation may be developed in Scotland.”

The Netherlands and other jurisdictions publish clear rules on how much cannabis a person can carry without the risk of prosecution.

In Scotland, the guidelines are secret, and police officers can also exercise their discretion.