The Government's advisers admit they took too long to ban mephedrone, allowing dealers to build up a stash of the drugs.

Professor Les Iversen, the chief drugs adviser, said they would need to act more quickly with the introduction of new temporary banning orders aimed at getting legal highs off the streets.

Problems with the drug were highlighted in September 2009, but it was not banned until March last year.

Professor Iversen said: "If we take too long about our deliberations it gives dealers an opportunity to stockpile that drug during the period while it's still legal."