The herbal stimulant drug khat is to be banned, the BBC understands.
The decision goes against the recommendation of the Advisory Council on the Misuse of Drugs (ACMD), the government’s official advisory body.
In January the ACMD said khat should remain a legal substance, saying there was “insufficient evidence” it caused health problems.
But Home Secretary Theresa May has decided to ban it, BBC home affairs correspondent Danny Shaw said.
An official statement is expected shortly.
Mrs May’s decision is thought to be based on wider security and international considerations, in particular the use of the UK as a transit route for khat to other European countries, our correspondent said.
Khat is traditionally used by members of the Somali, Yemeni and Ethiopian communities.
It is already illegal in most European countries, as well as the US and Canada.