Former deputy prime minster Nick Clegg and former health minister Norman Lamb joined Labour and Tory figures to back a new report which claimed legalisation could net the Treasury more than £1 billion a year in tax revenue.
The Adam Smith Institute and Volteface study argued that the UK should follow the lead of the United States, where four further states legalised marijuana in this month’s elections.
It came as Germany gets ready to legalise cannabis for medical purposes and Canada prepares for all-out decriminalisation.
The Netherlands effectively decriminalised cannabis decades ago while Portugal legalised it in 2001.
The report argued that Britain’s “dark ages” drugs policy has failed to stop people drugs being manufactured and used, and to stop associated crime, corruption and killing.
It called for “root and branch” reform to legalise and regulate cannabis to ensure it meets acceptable standards, and to remove the market for criminal gangs.
A legal cannabis market could be worth £6.8 billion to the economy annually, potentially netting between £750 million and £1.05 billion in tax revenues and reduced criminal justice costs.
The number of offenders in prison for cannabis-related offences in England and Wales would also likely drop from the current 1,363, who cost taxpayers £50 million a year, the report said.
Mr Clegg said: “British politicians need to open their eyes to what is happening in the rest of the world.
“Cannabis prohibition is being swept away on a tide of popular opinion and replaced with responsible legal regulation.
“Now is the time for ministers to start writing the rules for this legal market, including age limits and health warnings, so that we can finally take back control from the criminal gangs.”
His Liberal Democrat colleague Mr Lamb said: “Prohibition is harmful and counter-productive, helping neither to reduce drug use nor the risks to public health. While other countries and US states are increasingly coming to adopt a more enlightened approach to drug policy, we are stuck in the dark ages, filling the pockets of criminals and perpetuating the stigma which prevents so many drug users from seeking help.”
Conservative former cabinet minister Peter Lilley said: “It is time we legalised cannabis. Currently cannabis can only be obtained from illegal gangs who also push hard drugs. So we are driving soft drugs users into the arms of hard drugs pushers.”
Conservative MP Michael Fabricant said: “There can be no doubt that just as prohibition on the sale of alcohol failed in the United States and encouraged gangsterism, the banning of drugs has promoted a wicked and lucrative black market which pushes illegal drugs on the innocent.”
Labour MP Paul Flynn said: “The UK’s 45 years of harsh prohibition has multiplied use and harm.
“A legal market would destroy the drug’s attraction as forbidden fruit and encourage users to ingest cannabis, of known strength and quality, in ways that will the avoid deadly dangers of smoking.”
Co-leader of the Green Party, Caroline Lucas said: “The War On Drugs has been an abject failure, and the continued criminalisation of cannabis users is deeply counter-productive.
“Britain needs an evidence-based drugs policy, rather than continuing the expensive and ineffective prohibition regime.”