Drug campaigners have backed the warning from police chiefs that new government powers to ban legal highs will not work.
The UK Drug Policy Commission (UKDPC), which analyses drug laws, said simply adding to the long list of substances already banned "won't make much difference".
Roger Howard, the UKDPC's chief executive, said: "We are deluding ourselves if we think that the temporary ban will solve the problem."
It comes after the Association of Chief Police Officers (Acpo) said the solution to tackling legal highs does not lie in "adding inexorably to the list of illicit substances".
The police chiefs questioned "the extent to which legislation can realistically be used to address active choices being made by (predominantly young) people".
Mr Howard said: "It's right for the government to react quickly when a worrying new drug emerges.
"But as Acpo have said, just adding the drug to the long list already controlled won't make much difference.
"The police and forensics are under too much pressure already to be able to offer much deterrent to potential users.
"We are deluding ourselves if we think that the temporary ban will solve the problem."
He went on: "We should think instead about what other powers we can use. Trading standards controls could provide a boosted first line of defence.
"We should encourage retailers to work with the authorities to reduce the damage that drug use can cause, and allow us to bring some discipline to an unregulated market."
Even the government's own drugs advisers have concerns over the new powers, saying they hope a better way of tackling legal highs could be found.
Professor Les Iversen, chairman of the Advisory Council on the Misuse of Drugs (ACMD), said: "Picking them off one by one is not necessarily very productive."
As one substance is banned, another one is produced which has similar effects but which is designed to avoid the scope of the ban, he said.
"That happens all the time.
"Hopefully we can find a better way of addressing the problem, rather than just hitting the compounds one by one."
Speaking at a public meeting in central London last week, he said consumer protection legislation and the Medicines Act 1968 could be used instead.
And he warned that the committee could become overwhelmed if too many legal highs were banned by the Government.
The row follows Acpo's submission to the Commons Home Affairs Select Committee inquiry into the government's drug policy.
The document, seen by The Times and confirmed by Acpo, said: "From an early stage, the chair of the Association of Chief Police Officers' (Acpo) drugs committee was of the opinion that the solution to the particular challenge of legal highs did not lie in adding inexorably to the list of illicit substances.
"A key question for the government to determine is the extent to which legislation can realistically be used to address active choices being made by (predominantly young) people and to tackle the undoubted harms caused by the misuse of substances taken essentially for pleasure."
The police would continue to "focus their energies on serious criminality" and "take a less robust enforcement approach" when it comes to personal possession, it added.
Last week, mexxy, which is sold as an alternative to ketamine and has been linked to two deaths, became the first so-called legal high to be subject to the government's new banning powers.
It will be made illegal for up to 12 months while the government's drugs advisers consider whether it should be permanently controlled.
Mrs Bercow assured her more than 45,000 followers on Twitter she would not actually buy any, but admitted she was tempted and "now obsessed with the stuff, despite never having heard of it 1/2 hr ago".
"Am I the only one now slightly tempted to try mexxy before it becomes illegal? I won't, obvs," she wrote.
The ban followed concerns that two people whose bodies were found in Leicestershire in February might have taken some form of the drug after buying it over the internet.
Police warned people not to take mexxy, which was advertised and sold as a safe alternative to the class C drug ketamine, after the bodies of a 59-year-old woman and a 32-year-old man were found in Leicester and Melton Mowbray on February 11 and 12.
Under the new temporary banning order, anyone caught making, supplying or importing mexxy, or methoxetamine, could face up to 14 years in prison and an unlimited fine under the Misuse of Drugs Act 1971, the Home Office said.
Simple possession will not be an offence, but police and border officials will be allowed to search or detain anyone they suspect of having the drug and seize, keep or dispose of a substance they suspect is mexxy.
A Home Office spokeswoman said: "The UK is leading the way in cracking down on new psychoactive substances by banning them while the harms they cause are investigated.
"Drugs ruin lives and cause misery to families and communities.
"Our strategy is to keep drugs off the streets and punish the dealers."
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