A group of scientists has urged the House of Lords to listen to scientific advice rather than the ranting of Home Secretary Jacqui Smith and reject her proposal to change the classification of cannabis from C to B.

In a letter to the Guardian , eight leading scientists call on the Lords to back an amendment tabled by Baroness Meacher which would delay any reclassification until the issue is re-examined.

The letter said: "Cannabis use has fallen in recent years, especially following its downgrading to class C in 2004, and it is obviously unwise to risk reversing that trend. The classification system must be credible - reclassification would send out an ambiguous message about the dangers of current class B drugs.

"Even more importantly, the move would be a sad departure from the welcome trend - established after the Phillips report into the BSE disaster - of public policy following expert scientific advice unless there is new evidence."

The letter was signed by an ex-government chief scientific adviser David King, Professor Michael Rawlins, ACMD chair from 1998 to 2008, Robert May, another ex-government chief scientific advister, Professor Gabriel Horne, chair of the Academy of Medical Science working group on addiction and Professor Colin Blakemore, a member of the UK Drug Policy Commission.

Smith's scientific advisers - the ACMD - investigated alleged links between cannabis and mental illness and found there weren't any. Despite this and evidence that cannabis use has fallen since it was downgraded by David Blunkett in 2004, Smith said she wanted to upgrade the drug and ban the sale of paraphernalia like pipes and bongs.

Even the Home Office press release at the time said: "Cannabis use has fallen significantly across all age ranges and this is a testament to the success of the previous ten year Drug Strategy. However, the reduction in cannabis use must not be allowed to reverse."

The Association of Chief Police Officers also asked for flexibility in how it deals with people caught in possession of the drug. ®