Figures for the number of deaths caused by legal highs are misleading and hampering attempts to formulate a sensible drugs policy in the UK, former government advisers warn on Friday.

Prof David Nutt and Dr Les King say that the number of deaths, recorded as having risen 600% between 2009 and 2012, have been inflated by the inclusion of fatalities linked to substances that do not meet the definition of legal highs, or "new psychoactive substances".

In a letter published in the Lancet, the scientists from the Independent Scientific Committee on Drugs say that the figure of at least 68 deaths recorded by the National Programme on Substance Abuse Deaths (NPSAD) in 2012 included deaths from substances that are already illegal, not new and/or not psychoactive.

Writing in the Guardian, they claim that just 11 of the 68 deaths cited by NPSAD were from current legal highs. "What is certain is that if the current government review of legal highs is to be taken seriously and lead to health improvements then there must be a proper definition of terms and improved data collection," they say. "Moreover the data must be properly and independently audited so the effects of any change in the law can be properly evaluated."

They attribute 20 of the deaths in the NPSAD figures to p-methoxyamphetamine (PMA) and p-methoxymethyl- amphetamine (PMMA), which have been controlled drugs in the UK since 1977. Other substances implicated in deaths that the scientists say do not meet the definition of "legal high" include khat, which is not new, and anabolic steroids, which are not psychoactive.

They also claim that there are problems in data published by the Office for National Statistics on deaths related to drug poisoning in England and Wales. These included 52 deaths associated with new psychoactive substances in 2012, with 13 of them linked to γ-hydroxybutyric acid (GHB), which was made illegal in 2003.

Nutt was sacked as the government's senior drugs adviser in 2009 after criticising its decision to toughen the law on cannabis, and King resigned his role as a government expert in protest at the treatment of his colleague. They have set themselves up for another clash over drugs policy by questioning whether the published figures represent "sloppy science or whether there has been some attempt to massage figures to justify the current political focus and new review on legal highs".

NPSAD accepted some of the criticism and said it welcomed contributions to help improve its data, but it maintained that legal highs were a legitimate concern because more people are dying "than ever before". "Unfortunately, there is neither a universal definition of novel psychoactive substances (NPS) nor publicly available list of such drugs for researchers to work from," a spokesman said. "We would agree that there is an urgent need for a debate on this issue and clear definitions established and formally adopted."

"The section on NPS in our recent annual report describes trends over the period 2009 to 2012 in a range of emerging substances including former pharmaceutical or therapeutic drugs which could be misused and substances which have subsequently become controlled drugs."

An spokesman for the Office for National Statistics said: "ONS does not classify any drugs as 'legal highs'. The ONS annual bulletin makes this clear … ONS has not 'massaged' the figures for political purposes. It is an impartial organisation and subject to a strict code of practice."