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 as it came into force.
Danny Kushlick, director of Transform Drug Policy Foundation, said: "Yet again the Government has ignored the advice of its experts and prohibited another drug.
"As ever, it will serve to create a new income stream for organised crime and that insurgents could profit from.
"At the same time it will unnecessarily criminalise a minority group of Somalis and Yemenis, and deprive producers overseas of much needed legitimate revenue.
"It is high time that the legal regulation option was considered, not only for khat, but for other prohibited drugs."
In a written statement earlier this year, Theresa May said despite the recommendation of the Advisory Council on Misuse of Drugs (ACMD) not to ban khat, the body acknowledged that there was an absence of robust evidence in a number of areas.
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.
Mrs May said failure to take action in the UK would place the country at serious risk of becoming a single hub for the illegal onward trafficking of khat to countries where it is banned.
Chief Constable Andy Bliss, national policing lead for drugs, said: "Enforcement of the khat ban will be firm but proportionate.
"Officers will take into account the nature of the offence and its severity, using a tiered approach towards offences relating to possession for personal use.
"The police are working with Home Office colleagues, healthcare providers and community leaders to ensure that people in localities where khat use is prevalent are aware of the change in law and the police approach, as well as the support available to them."