Government plans to overrule its own drug advisers and reclassify cannabis as a more dangerous substance are attacked by leading scientists and MPs in a letter to the Guardian today.

The home secretary, Jacqui Smith, intends to move cannabis from class C to class B, where it will sit alongside amphetamines, such as speed, and barbiturates. The move comes despite repeated recommendations from the government's drug advisers that its classification should not be upgraded.

The proposal, which is due to be voted on by peers today, is described as "extremely damaging" in the letter, whose signatories include two former chief scientists, Sir David King and Lord May; Professor Colin Blakemore, former head of the Medical Research Council; and Sir Gabriel Horn, chair of the Academy of Medical Science's working group on brain science, addiction and drugs.

The letter warns that changing the classification of cannabis risks reversing a downwards trend in use of the drug since 2004 and undermines public health messages about the more serious dangers of class B drugs. It urges peers to block the change of classification by voting to defer the move until 2010.

"In recommending this change to parliament the government has rejected the explicit advice of its appointed experts, the advisory council on the misuse of drugs, for the first time in its history," the experts write.

The Lib Dem science spokesman Evan Harris said the letter demonstrated the anger in the science community over the government's treatment of expert scientific advice. "It may be that it will take resignations in order for ministers to understand that they can't ignore the evidence and keep scientists on board," he said.

The government's advisory council on misuse of drugs (ACMD) has reviewed the classification of cannabis three times since 2002. Its most recent report, which was commissioned last July amid concerns that highly potent "skunk" was becoming widely available, found that while stronger homegrown strains of the drug dominated the market there was only weak evidence of a link with mental illnesses, such as schizophrenia.

The report called for tougher action against cannabis farms, a crackdown on shops selling cannabis paraphernalia and a renewed public health campaign. Scientists on the council warned that reclassifying cannabis was unlikely to curb usage, but risked increasing the chances of vulnerable people getting a criminal record.

In a last-ditch attempt to block the Lords from approving the government's plans, Baroness Meacher has tabled an amendment that would postpone a decision on the drug for two years, pending another review by the ACMD.

In May Smith told the Commons that she had to take public perceptions and the pressures on policing into account when making a final decision on cannabis.