The chief drugs adviser to the government has given his strongest warning yet on legal highs in Britain, saying there are now more than 200 synthetic psychoactive drugs being sold outside existing laws.

Prof Les Iversen warned of the arrival of a new generation of compounds that imitate the effects of 1960s-style LSD psychedelics and cautioned that they could bring with them serious risk of overdose.

Iversen, the chairman of the home secretary's advisory council on the misuse of drugs (ACMD), said these untested legal highs were no longer "a nice set of party drugs that we can let people get on with". He added that sooner or later unexpected and serious harm would emerge from their use. The warning came at the ACMD's twice-yearly meeting to review its progress in tackling legal highs.

The Home Office has introduced temporary banning orders that outlaw the supply and sale but not possession of the drugs, pending an examination of their harmful effects.

But Iversen warned that the development of legal highs, often by chemists in illicit labs in China, was far outpacing the system. Two new drugs – mexxy (methoxetamine) and black mamba (a synthetic cannabinoid mix) – were banned after being categorised as class B last year.

The experts were also able to identify within a week to 10 days all the drugs being used at last year's Glastonbury festival, but the existing legal framework could not cope, Iversen said.

"The European monitoring centre for drugs and drug addiction logged 60 new psychoactive substances last year and a similar rate of one new compound a week so far this year," the ACMD chairman said.

"They now list 200 different psychoactive substances that lie outside our existing scope of regulation.

Prof Les Iversen said untested legal highs were no longer 'a nice set of party drugs that we can let the kids to get on with'. Photograph: Home Office/PA

"Our problem is to know how many of these are really being used in this country and how harmful they are. This is difficult because we can't possibly address all classes of compound at once, unless we and the government can think of clever ways of regulating."

He said they were particularly worried about the impact of a synthetic drug that is becoming widely available in Britain that imitates the hallucinogenic effects of LSD, which once fuelled the 1960s psychedelic era but has been out of popularity for more than 30 years.

Iversen said a dose of synthetic LSD could be measured in micrograms, which was so minute that it could not even be measured with an analytical scale. It was usually supplied in a diluted solution, as a drop on a piece of blotting paper, but it was also possible to buy it in powder, spray or fluid form.

"The dangers of an overdose are clearly immense. We are looking at it with a great deal of caution and worry," he said.

He added that the misuse of existing consumer laws, which has led to legal highs being sold as plant food, had caused them to be widely available.

"They are a set of chemicals that are potentially very dangerous. Novel psychoactive chemicals can be made in China one week and shipped to the UK for human consumption the next without any safety data.

"To me that is an appalling situation. Sooner or later we will get unexpected serious harms emerging from one or other of these compounds. We will then blame ourselves for letting them be sold without any safety precautions," warned Iversen.

He rejected a new approach in New Zealand, which tests and licenses the sale of these new psychoactive substances, as unworkable in Britain, but said a solution might be found by tweaking the Medicines Act or using consumer protection laws.