The Liberal Democrats will debate calls for a government inquiry into the decriminalisation of all personal drug use, at their annual conference in Birmingham next month.

Some party members support more emphasis on rehabilitation, treatment and education rather than prosecution. If the motion wins support, it will result in a change of Lib Dem policy.

The motion says that current laws on drugs are “harmful” and “ineffective” and call for drug users to be required to have counselling or treatment rather than facing jail or a fine.

It says that there is "increasing evidence that the UK's drugs policy is not only ineffective and not cost-effective but actually harmful, impacting particularly severely on the poor and marginalised".

It adds: "Individuals, especially young people, can be damaged both by the imposition of criminal records and by a drug habit, and… the priority for those addicted to all substances must be healthcare, education and rehabilitation, not punishment.

"One of the key barriers to developing better drugs policy has been the previous Labour government's persistent refusal to take on board scientific advice, and the absence of an overall evaluative framework of the UK's drugs strategy."

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Personal drug use has already been decriminalised in Portugal, and the Liberal Democrats would like to see a similar system in place in the UK. They aim to follow the Swiss lead to provide more clinics to help those addicted to heroin.

For an official government inquiry to be set up, the Conservative party would also need to back the policy, however Prime Minister David Cameron dismissed calls from Labour former Cabinet minister Bob Ainsworth to legalise class A drugs last year.