Personal possession of all drugs should be decriminalised, the Liberal Democrats announced today.

The policy, featured in the party's 'pre-manifesto', is understood by Politics.co.uk to be a first step towards the complete decriminalising of all drug use.

This morning's document, setting out the Lib Dems' priorities in the next parliament, promised "further work" to divert users "into treatment or other civil penalties which do not attract a criminal record".

While the document does not use the word 'decriminalisation', senior Liberal Democrat sources told Politics.co.uk the pre-manifesto was adopting "the essence" of a policy document proposal which called on the UK to follow the approach used in Portugal.

Portugal decriminalised personal possession of all drugs in 2000.

Nick Clegg, speaking at the pre-manifesto launch, told Politics.co.uk he had been a longstanding advocate that drugs policy in Britain has been "blighted by kneejerk prejudice".

"We should stop decanting hundreds of young people into prisons who have committed no other offence, but only been put into prison because of the possession of drugs for personal use," he said.

"The evidence is overwhelming: doing that is often the quickest, surest route to turn that individual into a hardened criminal and an addict using harder drugs.

"There are lots of other countries experimenting: states in the United States, in Latin America and Portugal which are doing a number of different things.

"What we'd say is we'll look at it and at what the evidence shows works."

The pre-manifesto confirmed Clegg's promise to end the use of imprisonment for personal drug use and pledged to "move the drugs and alcohol policy lead from the Home Office to the Department of Health".

It also sets out to establish a commission to assess "the effectiveness of current drug laws and alternative approaches".

The original policy document on which the pre-manifesto promise was made says the party will "adopt the model used in Portugal where those who possess drugs for personal use will be diverted into other services".

Under the system, police would decide whether someone caught with possession of a drug is a dealer or user. For those only using the drug, the onus would be on a medical response rather than criminal sanctions.

The move goes a step further than Clegg's previous pledge to stop sending people to jail for drug use.

It is the latest in a line of increasingly liberal positions on drugs from the party, as it becomes more confident in demanding wholesale reform of Britain's drugs laws.

Jeremy Browne, the party's former Home Office minister, and Norman Baker, its current Home Office minister, were both sent on international drug law fact-finding missions by Clegg.

The deputy prime minister has called for a royal commission on drug laws to be set up and for European countries to come up with a common position on drug legislation ahead of a UN special session on drug law in 2016.

The Lisbon system retains the illegal status of recreational drugs if the amount is no more than ten days' worth, but makes the offence an administrative - rather than a criminal - one.

Predictions of a rise in drug use failed to materialise, with studies suggesting it actually declined among 15-24-year-olds, the population most at risk of initiating drug use.

In the years leading up to decriminalisation the number of drug-related deaths in Portugal had soared, with rates of HIV, Aids, Tuberculosis, and Hepatitis B and C among those injecting drugs rapidly increasing.

Following the move the number of newly-diagnosed HIV cases among drug injectors declined dramatically, from 1,016 in 2001 to 56 in 2012. Similar trends were evident for Aids cases and Hepatitis C and B.

The Portugal experiment provided potent data for those who argued that the punitive quality of a country's drug laws had no relationship to its rates of drug use.

A policy modelled on the Portuguese example was adopted by the Liberal Democrats in their policy paper following debates with party members. It will now become part of the party manifesto in next year's general election.

Whether it would survive coalition negotiations is another matter. Neither the Conservatives nor Labour have shown any interest in reforming Britain's drug laws, although there are reports of a more sympathetic response from David Cameron in recent months.

The policy paper also recommends making the Advisory Council on the Misuse of Drugs independent and setting up a review of experiments in regulated cannabis markets in Washington, Colorado and Uruguay, although these policies did not make it into the pre-manifesto.