CANNABIS should be sold with cigarette-style health warnings and tax levies designed to deter users from purchasing the most potent strains, drugs campaigners have said.

The call came as a cross-party group of MPs predicted that the UK will completely legalise cannabis use within 15 years.

The claim followed a visit to Canada, which legalised recreational cannabis sales in August 2018, by Conservative MP Jonathan Djanogly, LibDem MP Sir Norman Lamb of the Liberal Democrats and Labour MP David Lammy.

Drug regulation is reserved and a spokesman for Home Office stressed that it has "no intention of changing the law".

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However, Mr Djanogly said he now believes it "will happen" within 10 to 15 years. Mr Lammy and Sir Norman predicted a legislative u-turn within five years.

Canadian lawmakers wanted to curb illicit cannabis use among teenagers, which was the highest of any G7 country, and crackdown on black market trade.

Cannabis of all strains and strengths is sold in quantities of 30g per purchase in specially-licensed dispensaries and, in some provinces, online. Buyers must be over-18, show ID and also sign for it in the case of home delivery.

Black market trade in cannabis is said to have halved since regulation came into force.

Paul North, director at pressure group Volte Face, which organised the MPs' trip to Canada, said the model was a "step in the right direction".

He said: "The UK market is dominated by high potency cannabis, and the police don't seem to be able to tackle the gangs making millions. What Canada is doing isn't perfect but it's far superior to what we have in the UK."

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Around 97 per cent of cannabis traded illicitly in the UK is high-strength. Mr North said it would be impossible to eliminate this from the market through regulation, but that harm could be reduced.

He said: "If you legislated to regulate cannabis sales but you banned high-potency cannabis from sale in the legal market, you're still going to have demand for it.

"In a regulated market, if you had health message on the packet, taxed it a bit higher so that lower potency versions were cheaper, then you can encourage people to buy less strong strains of cannabis.

"As much as there are concerns about high potency cannabis, many people use it and they're completely fine - it's a minority who have an issue, and right now the market is flooded with that type of weed."

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The Scottish Drugs Forum (SDF) has been campaigning to decriminalise the possession of all illegal drugs for personal use in the wake of figures revealing Scotland's drugs deaths are now the highest in the world.

David Liddell, chief executive of SDF, said there was "some sense" in also regulating the supply, not only for cannabis but all drugs.

He said: "It would also eliminate the inherent dangers of criminal supply in terms of varying purity and adulterants.

"We would support any measure that de-stigmatised people and prevented people who use drugs, particularly those who develop problems being marginalised."

A spokesman for charity, Addaction, said there were "many good arguments to regulate cannabis" but stressed that "any regulated model must put people’s health ahead of private profits.

He said: “First, we need to make sure a significant amount of the tax revenue raised is ring fenced to help people who run into problems with cannabis.

"Taxation should also be used to reduce people’s access to high strength cannabis, which has been linked to psychosis. This should be done in a similar way to minimum unit pricing, which sets the lowest price a unit of alcohol is sold for.

“Cannabis should be sold in plain packages with public health warnings in a similar vein to tobacco.”

A Home Office spokesman said: "The legalisation of these substances would not eliminate the crime committed by the illicit trade, nor would it address the harms associated with drug dependence and the misery they can cause to families and society."

A Scottish Government spokesman said: “Addressing drug related harms continues to be a public health priority for this government and we are prepared to consider innovative, evidence-based approaches, even if at first they may be challenging.

“Drug control is currently the responsibility of the UK Government under the Misuse of Drugs Act 1971.

“We believe responsibility for this area should be devolved so future decisions on these issues would be for the Scottish Parliament to consider.”