Experts have backed calls to be made at the Liberal Democrats conference this weekend for the decriminalisation of all drugs, saying it would not lead to a surge in drug use.

The UK Drugs Policy Commission, which includes Professor Colin Blakemore, former chief executive of the Medical Research Council, and Dame Ruth Runciman, former chair of the Mental Health Act Commission, among its membership, says it backs the broad thrust of the Lib Dem motion to be debated on Sunday.

The motion calls for all criminal penalties for personal possession to be scrapped, the introduction of a regulated market in cannabis, and the expansion of heroin maintenance clinics for the most fervent users.

Ewan Hoyle, of Liberal Democrats for Drug Policy Reform, who is to move the motion on Sunday, said it was likely to be passed, and welcomed the commission's endorsement.

Roger Howard, the commission's chief executive, said there was an understandable worry that removing criminal penalties for simple possession could lead to a rise in drug use, but insisted the move could do some good.

"The evidence from other countries suggests there would be no great surge in drug use," he said. "It is not so much the law that changes behaviour but rather social and cultural factors. This is why in the UK we have seen a decline in cannabis use over the past 10 years, despite various changes in the law and policing practice."

Howard said giving drug users a police record made it harder for them to get a job and put some off seeking help. "More challenging is what to do with people whose drug dependency results in them committing crimes to feed their habit," he said.

Existing drug laws had not put off 10 million people from using cannabis, Howard said. But the commission voiced caution about introducing a regulated market in cannabis. "Even in a tightly regulated market there is a risk of smuggling and counterfeiting," he said.

The Liberal Democrats last debated drugs policy in 2002, when the party voted to legalise cannabis and end prison sentences for the possession of cocaine, heroin and other drugs.

Hoyle said he hoped the party would take pride that its internal democracy allowed the membership to select the motion for debate without being vetoed by concerns about "reactionary tabloid hysteria".

"I do not see this motion as proposing radical reforms or as liberalisation of the drug laws," he said.

"I believe we have to fight a war on drugs as the harms they cause to families and society are so grave. This motion merely calls for consideration of whether the weapons that have been effectively deployed in other countries should be incorporated into the UK arsenal, and for the UK to lead the way in considering further restricting the harms drugs can cause through strictly controlling them."