“We are very angry with you British,” said the man I’d come to see at the Ministry of Agriculture in Nairobi. “We’re trying to work out how we can retaliate.”

I assumed he was referring to either the use of torture by the British during the war against the Mau Mau, highlighted by a recent court case; or to the British prime minister (like President Obama) going out of his way to avoid meeting Kenya’s new president, Uhuru Kenyatta, at a recent international gathering in Senegal. Kenyatta is soon to appear at the International Criminal Court, where he will be charged with crimes against humanity for his (alleged) role in the post-election violence in 2007/08.

In which case, why the wry smile and welcoming handshake from the man at the Ministry?

“I am referring to miraa,” he continued. “You know that your government is banning its use, don’t you? That’s very bad news for our farmers.”

It is some 30 years since I chewed miraa – or qat, as it is more commonly known in Britain, where some 90,000 Somalis, Ethiopians and Yemenis use it as a herbal stimulant. For them its consumption is part of a social ritual, much as an early evening glass of sherry was for my parents’ generation in middle-class England. Miraa is also used by African lorry drivers to help them stay awake – the active ingredient in Catha edulis is a mild form of amphetamine – and that’s how I first came across it. On a ride south from the Ethiopia border I chewed away at a bundle of bitter sticks, given to me by the driver, for about an hour, with little effect. It’s a drug that requires patience and a strong jaw.

A couple of days after I’d met the man at the Ministry, I travelled to Meru, four hours’ drive north of Nairibi. Kenya’s miraa farmers are concentrated here, in the heaving countryside on the leeward side of Mt Kenya. My companions and I did what most travellers do on a long journey: we whiled away our time by trawling through the mostly uninspiring pages of the local newspapers, the Standard, the Nation and the Star, which had much to say about the British ban.

It was, they reckoned, unreasonable. In declaring a ban on the import and use of miraa, the British Home Secretary, Theresa May, had gone against expert opinion. The UK government’s Advisory Council on the Misuse of Drugs had reported in January that there was insufficient evidence to suggest that miraa caused significant health problems, or was linked to organised crime, or helped fund terrorist activities.

So why has Mrs May decided to classify miraa as a class C drug, with the prospect of arrest and prosecution for persistent users? This is what she said: “The government will ban qat [miraa] so that we can protect vulnerable members of our communities and send a clear message to our international partners and qat smugglers that the UK is serious about stopping the illegal trafficking of qat.” Leaving aside the fact that individuals and companies who import mirraa have been doing so legally – they are traders, not smugglers or traffickers – what exactly does the claim that this “will protect vulnerable members of our communities” actually mean? She didn’t say.

Some observers in Britain have suggested that she was kowtowing to US pressure – miraa is banned over there – and following in the footsteps of the impeccably liberal Dutch, who have also banned it. The op-ed writers on the Kenyan newspapers suggested a darker reason: banning miraa in the UK would chime well with the anti-immigration lobby. A US-based Kenyan blogger in the Star surmised that the ban was a way of Britain showing its disapproval of Kenya’s new rulers.

This may be nonsense. But I sympathise with the Kenyans – all the more so having been to Meru, where several thousand farmers could suffer as a consequence of the ban. When Islamic rulers in the Somali capital of Mogadishu – ideologically marooned on the wilder shores of radical Islam – banned the import of miraa from Kenya a couple of years ago (the ban was later lifted) miraa exports fell by 40%. The UK ban will not only criminalise tens of thousands of British citizens, it will undermine the livelihoods of many Kenyan farmers.

Which leaves me with two thoughts. First, what’s the point of taking expert scientific advice if you’re going to ignore it, as Mrs May has? Her action strikes me as being both illiberal and irrational. Second, if the ban is to go ahead, couldn’t the UK Department for International Development – one of the few ministries whose budget has been ring-fenced – provide support to help Meru’s miraa farmers wean themselves off the crop and adopt other forms of horticulture or agroforesty?

The claim in the Star that the Kenyan president should be willing to cut deals – with Britain, presumably, but what deals? – “to save an entire tribe from economic annihilation and extinction” is obviously absurd. But I think there is a good case for doing what the United States has done, and continues to do, in parts of Latin America. The war against coca, the plant which provides the raw material to make cocaine, has involved, among other things, providing farmers with incentives to replace coca with crops like cocoa and oil palm.