Every day for seven hours, Yusuf chews the leaves of the east African shrub, khat. It keeps him in an amphetamine-fuelled haze for most of his waking life.

It begins at four o'clock in the afternoon when Yusuf, a Somali refugee, makes his way to a khat house, just off Mile End Road in east London. It's a spartan, quasi-caf with a few chairs, a makeshift bar serving water and little else - just a room in someone's flat. Bunches of khat are spread out on a newspaper in front of the eight men, who have left their shoes at the door. It is one of at least 100 khat houses, or 'peace rooms', in London. Others include a barber's basement, coffee shops and back rooms in grocery stores.

Without ceremony, the men start chewing. A wave of goodwill slowly rises across the room. It crashes around eight o'clock. The evening peters out a few hours later amid disinterested gibber and looming hangovers.

Khat contains cathinone, a naturally occurring amphetamine-like drug. It enters the central nervous system by seeping into the blood capillaries in the cheek, where it is held in a tight mush. As with synthetic amphetamines (speed), khat dehydrates users. Even before the drug 'kicks in' - about an hour - the men have a powerful thirst. By nightfall they will have drunk several litres of water each and eaten nothing.

'In Somalia I was a government official,' Yusuf says. 'It will not get me a job here. I have nothing to do and I chew khat more. At home I chewed most days. It does no more harm than cigarettes and alcohol.'

Britain's 35,000 Somalis, about 75 per cent of whom live in London, were well-educated, well-connected professionals back in east Africa - the ones who had the wherewithal to flee to the West. Now 90 per cent of them live on income support of 45 per week, and khat traders are among the few making money.

Yusuf's mornings and early afternoons are listless. His hangover barely lifts in time for the journey down to Mile End Road and the next chewing session. He is married with two children and his wife is thoroughly fed up with her husband's habit. His sex drive has come to a standstill and he looks painfully thin. Other possible side effects of his routine include psychosis, paranoia, schizophrenia, cerebral haemorrhaging, migraines, cirrhosis of the liver and gastric conditions. Yusuf could not care less. He is a Somali in London and khat makes him feel at home.

The plant is properly known as Catha edulis, and can grow to 6ft high. It has been harvested and consumed in the south-west of the Arabian peninsula and east Africa for at least seven centuries. It does not surprise Yusuf, but astonishes various Western governments, that khat is legal in Britain.

Last year the United States upgraded the drug from Schedule IV to Schedule I, alongside heroin and crack. Other countries to have outlawed it include Italy, Morocco, Switzerland, Canada, Norway, Sweden, Finland and Saudi Arabia. Britain is the only country with a sizeable Somali community which has not criminalised khat.

A 1980 United Nations report damned the drug as physically and psychologically ruinous. Khat, also known as Bushman's tea, la salade, and by some 30 other names, has been blamed for rotting the social fabric of east Africa, in particular Somalia, where the civil war is said to be 'khat-driven'.

Until recently, khat merely symbolised a culture clash between the First and Third World. But now the tentacles of a global smuggling network are slowly forming along the paths of the Somali diaspora created by the war. The trade is said to be worth 50m already, and London is the nerve centre.

About seven tonnes of fresh khat pass through Heathrow airport daily, legally brought over from Somalia, according to one dealer. Couriers are given a free flight and 100-150 for bringing two suitcases-full back to the UK. There are two basic strains: the stronger, Miri, from Kenya, is prized above Hereri from the Ethiopian region of the same name.

There is no crime associated with khat in Britain, where it costs only 4 a bunch (one to two are needed per session). This is still four times the price in east Africa, but between one-fifth and one-ninth the price in countries that have criminalised the drug. A substantial amount is consumed by Somalis in Britain (drugs workers estimate that 9,000 are khat addicts). The rest is shipped on to other Somali communities in the West.

For this, white, preferably female, couriers are recruited in pubs, via friends and through free classified ads. The couriers are informed that khat is a perishable delicacy, which in a sense it is; after four days out of the ground, the drug it contains loses its potency. It doesn't occur to the couriers that although they can legitimately leave on an international flight with khat, they often cannot lawfully disembark at the other end.

But sentences for unwitting couriers are set to increase abroad, especially since the trade has been linked to the trafficking of other illegal drugs and the supply of weapons to Somali warlords. US Customs expect to seize around one tonne of khat this year. However, it is Norway that has arrested the greatest number of khat smugglers from Britain.

Raymond and Joanne Edwards from south London are the latest couriers to return home from a Norwegian jail. They received the customary four weeks' detention followed by deportation. Had they reached their final destination, Sweden, it could have been six months. They were paid a mere 50 each. As yet, the Norwegians are understanding about the couriers' naivety, but impatience is building. There have been 15 cases in the past 12 months. The Norwegian prosecutor had asked for an exemplary four-month sentence for the Edwardses.

'There was another arrest the same day as the Edwards and another the next week,' a spokesman for the British embassy in Norway said. 'The Norwegians have told us in confidence that they are getting a bit fed up with this and might up the sentence. They really don't know what to do about it. They were saying, basically, 'why can't Britain make this stuff illegal?' '

Perhaps it is commendable British pluralism to allow Somalis to import their traditions. Or perhaps it is latent racism, ignoring a festering addiction because it is largely contained within an ethnic minority. The Somalian Association has lobbied the Government to have khat criminalised. Many wives feel that their husbands' lives now revolve around khat. Also, in a huge break with tradition, many Somali women have taken to chewing khat since arriving in Britain, though at separate sessions from the men.

'One argument for not slapping a control on khat is that it would arouse interest,' a Home Office spokeswoman said.

'Unlikely,' contends Janine, a British woman who has taken most types of illegal drug. 'It practically works your jaw off just to get a mild, buzzy feeling, and an oh-so-faint euphoria. It doesn't suit the Western pace of life. We want our hits there and then. This Third World chewing deal is too time-consuming.'

In the US a synthetic version called cat, a sort of extremely high-powered speed, is already available and proving popular. If a method could be found for extracting cathinone directly from the khat plant - similar to the simple refining process that yields cocaine from the coca plant - then khat could have potential for flooding the moneyed white market. The Home Office might react then, but the smuggling channels will have been thoroughly carved out. And, as the UN forces recently discovered, khat is grown in countries that are even harder to control than Latin American states.