The government's drug experts will tomorrow recommend banning Spice, a herbal smoking mixture thought to be as strong as some strains of skunk cannabis.

The decision, which the home secretary, Alan Johnson, is expected to endorse, will mark the first official move to curb a burgeoning market in "legal highs". Sales of herbal drug subsitutes have grown rapidly through a network of online sites and backstreet "head shops".

The government's Advisory Council on the Misuse of Drugs (ACMD) is to warn that Spice Gold, which is advertised as a herbal high and an "aromatic potpourri", is in reality far from innocuous. They will say it contains synthetic chemicals that mimic the effects of some of the more powerful active ingredients in cannabis.

The council's advice follows a request from the former home secretary Jacqui Smith to investigate the product in March.

She voiced her concern over the "wide and largely unregulated market in the sale of psychoactive legal alternatives to illegal drugs, particularly as they are actively marketed to young people in head shops and via websites.

"Advice on the availability and harms of these so called 'legal highs', with a particular focus on protecting young people, will be very useful in informing future government policies," Smith told the ACMD.

Spice Gold has been around since 2006, when it was first imported from China. The smoking mixture costs £20 for a 3gram pouch, and contains mostly unidentified herbal matter, with ingredients such as dried flowers, leaves and aroma extracts listed on the packet. It is sold in various"flavours", with Arctic, Diamond and Silver promising different strengths.

The Trojan Horse properties of Spice were only identified last December, by the THC Pharm laboratory in Germany, which is developing medicinal cannabis. The research led to a ban in Germany and Austria in January this year. France followed suit in February.

Martin Barnes, the chief executive of the drugs information charity DrugScope, said making Spice products illegal would be a "pre-emptive measure", although the limited available research did suggest there were "potential harms" attached to their use.

"Exploring control options for the substances could remove the incentive for the manufacture and supply of Spice, as it would no longer be available as a 'legal' alternative to cannabis," said Barnes. "While consistent with the aims of the Misuse of Drugs Act, a recommendation to make Spice illegal would be a pre-emptive measure and unusual in that it would be based on evidence of potential harms to individuals and society rather than proven actual harms."

Professor Leslie Iverson, chairman of the ACMD committeethat drew up the report on Spice, stated his concerns: "It's a very clever product, sold as a herbal smoking mixture from China, but containing chemicals which can be a lot more potent than cannabis," he told his local paper, the Oxford Mail. "Users have no idea what they are taking. As a result, they are running a considerable risk of overdosing, which is not only unpleasant but potentially quite dangerous."

It is expected that the ACMD will move on, after tackling Spice, to consider the position of other "legal highs" including Salvia divinorum, commonly known as magic mint or Mexican sage. An official consultation over plans to ban two synthetic party drugs, GBL and BZP – also known as "herbal ecstasy" – is due to end on Thursday.