Gordon Brown's desire to see cannabis reclassified as a Class B drug has received a setback after it emerged that the Government's drug advisory body has decided to recommend it stays in the less serious Class C category.

The Advisory Council on the Misuse of Drugs (ACMD) decided at a private meeting yesterday that it will not recommend reclassifying the drug, after 20 of its 23 experts voted there was not sufficient new scientific evidence to justify a change, The Times has learned.

If confirmed, the decision puts the council on a collision course with the Prime Minister, who this week spoke of his determination to send a signal to young people that the use of cannabis was “unacceptable”.

Mr Brown indicated his strong support for reclassifying cannabis as a Class B drug as recently as his monthly Downing Street news conference on Tuesday.

After personally asking the council to reconsider the drug for the second time in two years, the ACMD decision presents Mr Brown with an embarrassing political dilemma.

He must now decide whether to overturn the advisory council's decision and reclassify the drug on political rather than scientific grounds. It would be the first time in decades that politicians have rejected the findings of the Advisory Council’s panel of experts.

If, on the other hand, he decides to accept the council's advice and keep cannabis in Category C, he runs the risk of being accused by anti-drugs campaigners of being a ditherer who backed down on a promise.

David Davis, the Shadow Home Secretary, said that the Government's cannabis policy had descended into chaos.

"This shambles has been caused by Gordon Brown's complete inability to make a decision," said Mr Davis.

"Rather than take action, he took the soft option of ordering a review. He has allowed the expectation that he will reclassify to gather momentum – now he must act.

"With increasing amounts of skunk cannabis on our streets, more people needing hospital treatment and fewer people being prosecuted we need to reclassify this drug today. All Gordon Brown offers is more dither and delay."

Professor Sir Michael Rawlins, the chairman of the ACMD, has refused publicly to confirm the committee's decision, which was initially leaked to the BBC. Prof Rawlins said only that his committee's report would be sent to the Home Secretary this month.

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