Scientists are calling for three new synthetic drugs to be made illegal, after one was linked to 12 deaths in the UK.

The three forms of benzodiazepines, which would previously have been known as legal highs, are often injected alongside heroin to extend the effects of the drug, but can cause people to stop breathing.

Flualprazolam, flunitrazolam and norfludiazepem are also used as “come downs” and have been linked to suicidal thoughts in teenagers and people with alcohol dependence.

The board, who advise the Government on drug use, now want all three to be outlawed as Class C substances after a dozen deaths were linked to flualprazolam.

The classification would mean those found supplying or producing could get up to 14 years in prison, while two-year sentences could be handed down for possession.

Other benzodiazepine drugs are used to treat conditions including anxiety and insomnia, but the three recommended for classification are not used in any UK medicine.

Professor Owen Bowden-Jones, chair of the Advisory Council on the Misuse of Drugs (ACMD), said in a statement: “To prevent further health-related harm, the ACMD recommends the three benzodiapzepines are classified under the Misuse of Drugs Act.

“We are also concerned about the ongoing risk of other uncontrolled novel benzodiazepines.

“These compounds must be carefully monitored to see if they become more available in the UK drugs market and cause danger to health.”