Health researchers have claimed that those who experiment with legal highs are "dancing in a minefield" as they highlight a leap in the number of recent deaths in Britain connected with the synthetic drugs.

Experts who compile the annual report of the National Programme on Substance Abuse Deaths based at St George's, University of London, say the number of deaths in which legal highs such as Meow, Meow and Benzo Fury, have been directly implicated have risen from 10 in 2009 to 68 in 2012.

The increase in the number of deaths in which legal highs were identified as the cause of death comes as the overall number of drug-related deaths has tumbled from more than 2,000 to 1,613 over the same period.

The same researchers say there has also been a big rise in the number of times legal highs has been mentioned as a cause of death along with alcohol and other drugs, from 12 in 2009 to 97 in 2012.

The figures were released as the government announces the latest move in efforts to curb drug misuse, with tougher penalties for the use of ketamine, the horse tranquilliser.

It is expected to be moved from a class C to a class B drug, meaning illegal possession could lead to a five-year jail sentence, after the Advisory Council on the Misuse of Drugs warned that frequent use could cause "severe and disabling" bladder damage. Frequent use has led to some people having their bladders removed.

Prof Fabrizio Schifano, a spokesman for the NPSAD, said: "We have observed an increase in the number and range of these drugs in the postmortem toxicology results and in the cause of death of cases notified to us. These include amphetamine-type substances, dietary supplements, ketamine derivatives, among a host of others.

"The worrying trend is that these types of drug are showing up more than ever before. Clearly this is a major public health concern and we must continue to monitor this worrying development. Those experimenting with such substances are effectively dancing in a minefield."

Legal highs are synthetic or designer drugs that are chemically designed to mimic the effects of more traditional substances such as cannabis and amphetamines but whose chemical composition is not banned by existing drug laws.

The report comes after Norman Baker, the crime prevention minister, began a review of legal highs that will consider widening legislation to boost police and law enforcement agencies' powers.

"I am determined to clamp down on the reckless trade in so-called legal highs, which, as this report shows, has tragically claimed the lives of far too many people in our country,", he said.

In many cases traces of multiple legal highs were found with alcohol, suggesting that young people are mixing substances.

The report indicates an increase in the proportion of deaths involving stimulants such as cocaine and ecstasy-type drugs, following a decline in 2009. The report suggests that one reason for the increase may be a rise in the purity of powder cocaine and ecstasy tablets in 2012.

Overall, the number of drug-related deaths reported to the NPSAD fell from 1,757 cases in 2011 to 1,613 in 2012.

• This article was amended on 12 February 2014. The original version wrongly stated that the report was written by experts based at St George's hospital in London, rather than St George's, University of London.