A blanket ban on "legal highs" has been delayed until May.
The Psychoactive Substances Act was expected to come into force from next Wednesday.
However, the Government now expects the legislation to be implemented "in the spring".
Parliament will be given at least 21 days' notice before it starts, and given that Parliament is in recess until April 11, May 1 is the earliest possible date for the law to come into force.
In a written answer issued last week, Home Office Minister Karen Bradley said: "We expect to commence the Psychoactive Substances Act 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."
The Act will bring in a blanket ban on the production, distribution, sale and supply of "designer drugs", with sellers facing up to seven years in prison.
Last week it was confirmed that poppers will not be banned in the crackdown.
The Home Office said that while April 6 was the first date on which the legislation could be "commenced" or started, it was always subject to consideration of all the moves necessary across the UK for the provisions to come into force.
A spokesman for the department said: "In line with the advice of the Advisory Council on the Misuse of Drugs, the Government is in the final stages of putting in place a programme of testing to demonstrate a substance's psychoactivity prior to commencement of the Act."