Two years after they were banned, legal highs are now being sold underground, a review has found.

Legislation was introduced in May 2016 following an increasing number of deaths linked to the psychoactive substances.

Today's report from the Home Office has also revealed the law has failed to prevent new drugs from appearing on the market.

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Alex Stevens, who is a professor in criminal justice at the University of Kent, said it was obvious this would happen.

He said: "This ban shows, once again, that you can't solve drug problems by banning drugs.

"Drugs are a social problem and you need social solutions to them.

"We predicted two years ago when the law was introduced that the markets for new psychoactive substances, formerly known as legal highs, would merge with the market for illegal street drugs and that harms would be concentrated amongst the most vulnerable groups, including prisoners and homeless people and that's exactly what this review shows has happened."

Several deaths in Kent have been linked to legal highs and there was a high profile campaign to ban them by the mum of Jimmy Guichard, 20, from Gravesend who died shortly after taking synthetic cannabis.

While today's report concludes there appears to have been a reduction in the use of legal highs among the general adult population, it says there has been no change among children.

The use of new psychoactive substances amongst homeless people is described as 'mixed' and highlighted as a particular problem within prisons.

Professor Stevens added: "The same people that are dealing cocaine, ecstasy, heroin are also selling synthetic cannabinoids and other substances.

"There are also people buying them on the dark net.

"The open websites that were selling legal highs in the UK have largely disappeared but they've been replaced by foreign websites and encrypted websites that people can access on the dark web."

Raids were carried out on shops in Kent found to be selling legal highs in 2014, two years before the new legislation was introduced.

Professor Stevens says the government needs to be doing much more to tackle drug use: "If the government, which it has done, implements policies which increase homelessness and overcrowd our prisons then we're likely to see increases in drug problems which we have done."

"We need to work with the people who're most at risk - so those are people who're homeless and especially people who're leaving prisons.

"The services we provide to people leaving prisons are absolutely shockingly inadequate and people are ending up homeless, on the streets and looking for some way to sooth their pain.

"This trade, I'm afraid, fills that gap."

If you are experiencing problems with drugs, you can get advice at www.talktofrank.com