Under a new law that came into effect on Tuesday, khat is now a "class C drug", making possession punishable by up to two years in jail and supply and production punishable by up to 14 years.
Khat is the leaves and shoots of the shrub Catha edulis, which are chewed to obtain a mild stimulant effect.
British Prime Minister David Cameron said in an article for the Somali website Hiraan Online that the move would protect "vulnerable members of society."
"What is most concerning is khat's social impact," he wrote, adding that it had been blamed for "family breakdown, unemployment, debt and crime links to the global illicit drugs trade."
Britain was also becoming the "khat smuggling capital of Europe", after most of the continent plus the United States and Canada banned the drug, said Cameron.
Around 2,560 tonnes of khat worth Ģ13.8 million ($23.4 million, 17.2 million euros) was imported to the United Kingdom in 2011-12, bringing in Ģ2.8 million of tax revenues.
Khat, also called miraa, has been chewed for centuries in the Horn of Africa. Its psychoactive ingredients -- cathinone and cathine -- are similar to amphetamines but weaker, and can help chewers stay awake and talkative.