Theresa May, the Home Secretary, has been warned by the European Union’s drugs agency that Britain’s new blanket ban on so-called “legal highs” may not work.

The European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) said it was “unlikely” that any new regime could stem the tide of the designer drugs, which emulate the effects of controlled substances such as cannabis or heroin.

It comes just weeks before the Government’s Psychoactive Substances Act, which received Royal Assent in January, is due to come into force.

More than 100 new legal highs – also known as “new psychoactive substances” – were recorded by the authorities last year and more than 560 are now being monitored by the EMCDDA.