The ban on legal highs that was due to come into effect on 6 April has been postponed for at least a month or more, the Home Office has said.

The Psychoactive Substances Act, which has reached the statute book, has been delayed following claims that its current definition of a psychoactive substance may not be enforceable by the police or the CPS (crown prosecution service).

The introduction of similar legislation in Ireland triggered a wave of closures of shops selling legal highs and online outlets. It was anticipated that the introduction of a blanket ban in Britain would have a similar effect. But there have been very few prosecutions in Ireland so far because of difficulties in proving whether a substance is psychoactive or not.

A senior Home Office minister confirmed that a British ban would not come into effect on 6 April as planned but would be implemented “in its entirety in the spring”.

“We need to ensure the readiness of all the activity necessary to enable the smooth implementation of the legislation across the UK and to support law enforcement in their ability to drive forward the legislation on commencement,” Karen Bradley told MPs in a Commons written answer slipped out on the day of the Easter recess.

The legislation aims to ban any substance intended for human consumption that is capable of producing a psychoactive effect, with a list of exemptions of substances in everyday use such as alcohol, nicotine and caffeine.

Hampshire police have written to shop owners and others selling legal highs to warn them that it is their responsibility to ensure their products are not harmful or banned substances.

A Home Office spokesperson said that in line with advice from its drugs experts on the ACMD “the government was in the final stages of putting in place a programme of testing to demonstrate a substance is or not psychoactive prior to commencement of the act”.

“The landmark Psychoactive Substances Act will fundamentally change the way we tackle these drugs and put an end to unscrupulous suppliers profiting from their trade. Our message is clear: offenders will face up to seven years in prison,” they added.

The call for testing legal highs in the workplace is at an all-time high as UK business struggles to cope with employees taking legal highs as a replacement for other illegal substances which are traditionally tested for during a workplace drug testing programme.