Possessing, selling and importing khat - a plant used as a stimulant by Somalian communities - is illegal from today.
Khat, which makes its users feel more alert, happy and talkative when chewed, is now banned as a class C drug despite advice from the Government's official advisers that it should not be classified.
Around 2,560 tonnes of khat, which is also favoured by Yemeni and Ethiopian communities, worth £13.8 million was imported to the UK in 2011/12, bringing in £2.8 million of tax revenues.
Drug experts and policy campaigners have condemned the ban, saying it will simply push khat sales on to the black market while "unnecessarily" criminalising certain minority groups.
However, the Home Secretary said the whole of northern Europe, most recently the Netherlands, and the majority of other EU member states have banned khat, as well as most of the G8 countries including Canada and the USA.