The sale of cannabis could be legalised and sold in UK high street shops if the Liberal Democrats win the general election.

The pledge, which will be unveiled in full in the party's manifesto next week, is one of the most radical policies on drugs ever put forward by a British political party.

Under the Lib Dem proposal, the sale of marijuana would be strictly regulated to reduce harmful chemicals and sale would be restricted to over 18s.

Liberal Democrat candidate Julian Huppert told Buzzfeed News that tax raised from sales will be spent on public health campaigns. He estimated that cannabis sales could raise up to £1bn ($1.3bn) a year in tax and would reduce the burden on prisons, the police and the NHS.

Huppert, a former Home Affairs Select Committee member, said that legalisation would force criminal dealers off the streets and would allow people to access cannabis for medicinal purposes more easily.

He described the Government's current approach to cannabis regulation as "a disaster for young people, whose mental and physical health is being harmed by an increasingly potent product".

"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," he said.

"The prohibitionist approach costs a huge amount of money, means we criminalise a large amount of people, and increases the harm. We spend a lot of money making people's lives worse. That cannot be correct."

Huppert said that he had never smoked a joint. "I'm really boring. I've never had one. For me it's a public health issue."

The Lib Dems have long campaigned to legalise marijuana. Former health minister Norman Lamb tried to introduce a bill to legalise the drug in March 2016, arguing that the current law was "outdated, harmful and well overdue for reform."