Home Secretary Theresa May has decided to ban the herbal stimulant khat.
The move is despite the Government's Advisory Council on the Misuse of Drugs (ACMD) concluding in January that it should remain legal.
The ACMD described khat as a "mild stimulant" and said there was "insufficient evidence" that it caused health problems.
However, the substance is banned in the US, Canada, and most of Europe.
May expressed concerns in a statement that keeping the substance legal would turn the UK into a hub for the substance.
"Failure to take decisive action and change the UK's legislative position on khat would place the UK at a serious risk of becoming a single, regional hub for the illegal onward trafficking," she said.
Khat will be regarded in the UK as a class C drug, placing it in the same category as anabolic steroids and ketamine. Possession of class C drugs carries a two-year prison sentence.