So-called “legal highs” are now being sold underground since a blanket ban stopped them from being openly sold in specialist shops, a new report says.

Street dealers are the main suppliers of the products, known as new psychoactive substances (NPS), which mimic the effects of drugs such as cocaine, ecstasy and cannabis.

Since the Psychoactive Substances Act introduced in May 2016 there has a “considerable reduction” in the use of the products, according to the Home Office review.

Some 31 shops have since closed down and 332 no longer sell them, while police forces recorded nearly 500 arrests up to December 2016 and almost 1,000 instances of products being seized up to March last year.

However the products can still be sourced online, on the dark web and in some shops, the report says.

There are also concerns that stronger and more expensive versions are being produced, with the most vulnerable people gaining access to them, while use in prisons continues to be widespread.

A “range” of evidence suggests that some vulnerable users have shifted from NPS to traditional drugs, although the scale is not known.

Spice, a synthetic cannabinoid, is the most commonly used NPS in prisons.

Offenders can face up to seven years in prison, while civil orders have been used to shut down specialist shops and online sellers.

The 2016 legislation followed a spike in NPS-related deaths between 2011 and 2013, when numbers tripled from seven to 23, data from the Office for National Statistics show.

The substances gained popularity in the UK drugs market around a decade ago.

Martin Powell, from the Transform Drug Policy Foundation, warns that “little positive” has been achieved since the ban.

“This blanket ban was supposed to cure the UK’s ‘legal high’ problem, including Spice,” he said.

“But as experts warned before the new law was implemented, beyond the cosmetic success of ending legal sales in head shops, little positive has been achieved. And in many respects the situation has deteriorated.”