PLANS to legalise cannabis so it can be taxed and sold on the high street across the UK will be brought forward if the Liberal Democrats win power, the party has announced.

It is one of the first political parties to fight an election on a ticket of relaxing drug laws. Last year, the SNP party conference decided to back the decriminalisation of cannabis for medicinal use.

While Holyrood controls the issue of drug addiction and treatment, the legal status of drugs remains a reserved matter for Westminster.

The Lib Dem leadership said its proposal would take back control from criminal gangs, remove substances like “skunk” from the streets, and protect young people by introducing a legal, regulated market for cannabis.

Under the plan, cannabis would only be sold to people over 18 and sales would be strictly regulated; taxing the sale of the drug could generate up to £1 billion per year for the Exchequer.

The Lib Dems have long campaigned to legalise the drug, with Norman Lamb, the former Health Minister, attempting to bring in a Bill last year to tackle what he called "a catastrophic failure" of the war on drugs.

Julian Huppert, the party’s candidate in Cambridge, said the current approach to cannabis was "a disaster", which was causing significant mental and physical harm for young people.

He said: "There are no age checks and no controls on quality or strength. Skunk is widespread and the only ID you need to buy it is a £20 note.

"Successive governments have ceded total control of a significant public health problem to organised crime. The honest and pragmatic response is to take responsibility for this situation and regulate the market," he argued.

His concern was echoed by Mr Lamb, the party's candidate in North Norfolk, who said: "We are criminalising young people, people have no idea what they are buying so there is a major health risk.

"I am personally opposed to drugs and indeed to excessive alcohol consumption because of the health risks, and as a parent I absolutely want to discourage our adult sons from smoking or drinking or other harmful substances but the whole issue is what is the best way of dealing with this issue as the war on drugs has been a catastrophic failure."

People in possession of cannabis currently face up to five years in prison, while the class B drug carries a 14-year sentence for supplying it.

Last autumn, the SNP conference gave overwhelming backing to a motion which said drugs legislation, which includes the 1971 Misuse of Drugs Act, should be devolved to Holyrood and used to ease the prohibition of cannabis.

But the Home Office at the time made clear the UK Government had no plans to devolve drug control or legalise cannabis.

In a separate development, the Lib Dems announced it would commit to offering fathers an extra month of paid parental leave, to encourage greater sharing of parental responsibilities.

Jo Swinson, the former Business Minister, said: "Parents across the UK have already benefitted from greater flexibility and freedom in how they share the care for their new baby.

"But more needs to be done in order to encourage men to take leave when they become a dad, to bond with their child during the early weeks and months of their life."

The party said funding of the measure would be outlined in the Lib Dem manifesto, which is due to be published next week.