Khat has been legally imported into this country for 60 years, a mild natural stimulant chewed by a tiny slice of the population at hundreds of community cafes around the country. You may not have noticed these places, since they do not provoke the tension and violence associated with some pubs. But on Tuesday, the bitter-tasting plant became a banned class C drug – the latest example of the idiocy of the damaging war on drugs.

The impact will be felt largely among the Somali, Yemeni and Ethiopian communities, but we should all be concerned. For as 25 countries loosen drug laws and evidence grows from around the globe of the harm caused by prohibition in terms of lives lost and communities wrecked, this shows again how Britain is locked into a futile and backfiring battle that flies in the face of evidence, human rights and logic.

The decision to outlaw khat was taken last year by Theresa May, the home secretary. She ignored her own advisers on drug misuse who told her that it would be "inappropriate and disproportionate" to ban an innocuous trade that earns the Treasury a couple of million pounds a year in taxes. She brushed aside concerns from the Commons home affairs select committee, which concluded that it would make more sense to license importers than drive them underground.

Ministers admitted that it was hard to find evidence to back their ban; even the World Health Organisation says khat use carries low risk of harm. They ignored pleading – and a legal challenge – from Kenya, where farmers cultivating the herb in an impoverished corner of the country fear the decision will make their lives harder. Some tribal leaders called the act "a declaration of war" and threatened reprisals. Meanwhile the coalition boasts about its commitment to helping the Horn of Africa and curbing terrorism.

This myopic move comes as more progressive nations see that regulation is a more sensible solution than prohibition to the human desire to get high. After four decades, the war on drugs has cost hundreds of billions of pounds and thousands of lives. Anne-Marie Cockburn, whose teenage daughter died tragically from over-strong ecstasy, is the latest bereaved mother to jointhe campaign for reform; little wonder many doctors, police officers, intelligence officials and even politicians privately back her brave stance.

What will happen now? No doubt some people will stop chewing khat. Most traders in a thriving £15m-a-year sector will close down successful businesses, forcing scores of staff into unemployment. But others may carry on trading, joining the inevitable black market that springs up when something is banned. In the United States, where khat is already a controlled substance, it sells for 10 times its price on British streets; clearly, there will be hefty profits for any criminal gangs stepping in to meet demand.

As some MPs and community leaders have pointed out, asking the police to enforce a ban that only affects specific ethnic minorities also risks damaging race relations. There is a grave danger that outlawing khat risks further alienating sections of the Somali and Yemeni communities, already among the most marginalised groups in Britain and coming under increased pressure amid alarm over Islamic militancy.

There were claims of links between the khat trade and terrorism, but these seem tenuous. After all, al-Shabaab also bans its usage while the government's own drug experts have repeatedly said there is no evidence of criminal or terrorist involvement. They added, however, that they feared this might change following a ban; terror gangs have raised millions elsewhere in Africa by exploiting the drug market.

Such is the stupidity of Britain's latest salvo in the silly war on drugs. This will cost the country cash, put people out of work, increase communal tensions and may even help fund terrorism. One thing is sure: it will not terminate use of the banned substance.