Mr Bray said: "The law can't be outwitted in that sense any more because if it's psychoactive, it's psychoactive full stop.

"Therefore there is less incentive to have this sort of arms race-type approach whereby people are inventing new substances all the time simply to outwit what we do."

Although it was difficult to draw firm conclusions yet about the impact on use of the drugs, Mr Bray said: "Intuitively I think it must have had an effect. It will certainly make it less easy for the casual first-time user to get involved in this sort of stuff."

One of the key issues previously was the availability for people who would be "lulled into buying these things" thinking they were legal and therefore safe, and also accessible, Mr Bray said.

He added: "That aspect is now made much more difficult.

"Therefore there's a whole range of people who will not be persuaded to take these things anymore who previously would.

"So it's reducing danger for some."

Under the Act, offenders can face up to seven years in prison, while orders can be issued to shut down head shops and online dealers.

Safeguarding minister Sarah Newton said: "These dangerous drugs have already cost far too many lives.