A blanket ban on legal highs has come into force amid warnings it is "unworkable" and drug dealers will be able to escape prosecution.

The Independent has been told that concerns about the law’s enforceability - which reportedly influenced the decision to postpone its implementation from 6 April - have been greatly increased by the release of the Government’s ‘forensic strategy’ outlining the testing regime underpinning the Act. This was published, with relatively little fanfare, on 20 May.

Forensics experts, pharmacologists and lawyers say the ban will be "fraught with difficulty" and open to being defeated by "any self-respecting barrister".

Critics have already insisted the new law will just force users, who are often among the most vulnerable in society, to turn from ‘head shops’ to street dealers and the criminal underworld.

The substances, the most popular of which are synthetic cannabinoids sold under names such as spice and black mamba, are designed to mimic the effects of other drugs like cannabis and cocaine. But the cheap highs, which can be bought for as little as £10 per 1g packet, can be increbily potent. Some have described them as being worse than heroin.

As a result, they have gone from being almost unheard to constituting one of Britain's most serious problems in the space of just ten years. Last year, the chemicals were linked to more than 100 deaths in the UK.

The Government says the Psychoactive Substances Act will help eradicate the “abhorrent trade” in substances by introducing jail sentences for anyone selling or making a drug “capable of producing a psychoactive effect.”

Now, experts have claimed that under the ban, it will also be extremely difficult to secure criminal convictions against this new wave of drugs pushers.

After analysing the Governmet's forensic strategy released six days ago, Atholl Johnston, a clinical pharmacology professor at Queen Mary University of London who also runs a forensic testing laboratory, said: “Someone has been tasked with coming up with a strategy for an unworkable bill. It’s nicely written, but vague.”

He added: “We have had a lot of discussions within the various forensics groups, particularly the London Toxicology Group. We have had a lecture from a representative of the Home Office’s Centre for Applied Science and Technology. We are just not sure how this is going to work.”

The Government has come under huge pressure to deal with legal highs, which in 2013 had been tried at least once by 670,000 British 15 to 24-year-olds. Synthetic cannabinoids like Spice, Mamba or Insane can now be bought for as little as £10 per 1g packet.

But Prof Johnston said that securing a criminal conviction under the new ban would be “not easy at all.”

“There is a massive amount of uncertainty here,” he said. “How are we going to get the certainty [needed] to get a legitimate criminal prosecution?”

Of particular concern, he said, was whether the two in-vitro (test tube-based) tests outlined in the Government’s forensic strategy would be enough to a prove a substance was capable of producing a psychoactive effect to the criminal court standard of ‘beyond reasonable doubt’.

Similar anxieties had already been flagged up to the Government by the Advisory Council on the Misuse of Drugs (ACMD) before the Act became law.

In a letter written on July 2 2015, Professor Les Iversen, chairman of the ACMD, told Home Secretary Theresa May: “The psychoactivity of a substance cannot be unequivocally proven. The only definitive way of determining psychoactivity is via human experience… Such proxy measures [as in-vitro tests] may not stand up in a court of law.”

After receiving the Home Secretary’s reply, Prof Iversen wrote to her again, on 13 July, stating: “I would like to re-iterate that psychoactivity in humans cannot be definitively established in many cases in a way that would definitely stand up in a court of law where a high threshold of evidence is required.

“There is currently no way to define psychoactivity through a biochemical test, therefore there is no guarantee of proving psychoactivity in a court of law.”

The ACMD, Prof Iversen said, could only help in “formulating advice on how to predict that a substance is likely to be psychoactive.”

Echoing the ACMD’s concerns about in-vitro testing, Prof Johnston said: “Lots of things happen in the body that just don’t happen in a nice clean test tube system. Lots of things might interact with the drug before it can get to receptors [in the brain].

“For a start, the substance might not be absorbed into the blood properly. And will it cross the blood-brain barrier?”

He was backed by Dr Stephanie Sharp, a forensic pharmacologist and the co-founder of the Glasgow Expert Witness Service, who told The Independent: “Any self-respecting barrister could find an expert pharmacologist to counter any attempt to ‘prove’ psychoactivity using these tests.

“There are many instances of apparently active substances in-vitro that have no or a different effect in man.”

Legal Highs in Newcastle

The inability of existing law to keep pace with the rapid rate at which rogue chemists are making new legal highs has also prompted concerns about whether the Act will work in practice, with critics pointing to the example of Ireland, which introduced a similar ban in 2010.

The Irish ban has so far resulted in only a handful of prosecutions, while the European Commission has reported that between 2011 and 2014 Ireland experienced Europe’s second largest increase in legal high use among 15 to 24 year-olds.

A spokesman for the Irish Justice Department told The Independent the ban was enforceable, but admitted: “The emergence of new psychoactive substances happens at a pace that presents a challenge.”

With new legal highs now hitting European streets at an estimated rate of at least 100 a year, Rudi Fortson QC, a barrister who has specialised in legal high issues, said he doubted the UK ban would be able to keep pace.

He said the new law might appear to solve the problem by introducing a simple blanket ban on anything capable of producing a psychoactive effect, (with exemptions for food, medicines, alcohol, caffeine and tobacco). In practice, however, any new drug not already in the UK’s ‘library’ of Certified Drug Reference Standards would have to be tested before a criminal court could accept it was psychoactive.

“I would have thought that will be time consuming,” said Mr Fortson. “Given that drugs are altered so quickly and in so many different ways, will the reference standards be able to keep pace?”

The main problem with the legal highs ban, he added, was that “the principal issue of proving psychoactivity will be fraught with difficulty.”

Mr Fortson pointed out that the Government’s forensic strategy itself admits in-vitro testing “will not provide information on the potency of a specific compound”.

Although the strategy also insists the law does not require a determination on drug strength, Mr Fortson said defence lawyers might still be able to raise doubts about potency and query the inability of the tests to measure it.

“I don’t think the Government is entitled to water down what is meant by ‘capable of producing a psychoactive effect’,” he said. “If the Government means ‘an effect no matter how slight’ that is an absurdity. If the effect is so slight a user wouldn’t notice it, or if it just makes him blink a little more without getting high, is it proportionate to prosecute?”

“In terms of criminal offences,” he concluded, “there are real problems [with the Act]. The Government can expect these cases to be strenuously defended and any prosecution is likely to encounter significant difficulties.”

A Home Office spokesman said: “We have put in place a world-leading testing programme to determine whether a substance is capable of having a psychoactive effect (the evidential requirement under the Act). This is in line with [the final] advice from the Advisory Council on the Misuse of Drugs.

“The Forensic Science Regulator has endorsed the specification. We are confident psychoactivity can be established under the definition in the Act through this in-vitro test. It remains open for expert witnesses to draw on any other relevant evidence, including other research and studies.

“The Act will put an end to the game of cat and mouse in which new drugs appear on the market more quickly than Government can identify and ban them.”