The delivery trucks arrive with heavy loads and jostle for room on the grit and mud as men stand around or sit on forklifts, some chatting, some smoking, until the time comes to unload the boxes and stack them in rough rows in the warehouse.

The early morning lorries brought familiar foods to this depot in Southall, west London. There are aubergines, okra, and beans from Ghana. Other containers held herbs, or more exotic items, labelled jackfruit and breadfruit. But later a more controversial haul arrives: a load of qat that has just cleared customs at nearby Heathrow airport.

Thousands of boxes of the green, stringy plant are brought here each week for distribution across the UK. The plant goes to small retailers who sell it from vegetable shops, or to "marfashes", the places where mostly men, and many Somali, meet to chew and enjoy its mildly intoxicating effects.

On Wednesday morning, there was nothing illegal about the qat trade, and few at the depot thought a ban was coming. "They are always talking about a ban," said one Somali man in his 20s, who chewed qat from a blue plastic bag while sitting on a table where paperwork recorded the day's deliveries. "But if they ban it, I will go back to my home country," he said.

As more men gathered round, one grabbed some leaves from the bag and shoved them in his mouth. "Look. It's not bad," he said, munching with an open mouth. "Why would they ban it?" Neither man would give his name.

The workers wore casual clothes but another, in a suit, identified himself as the operations manager. He suggested a meeting late in the afternoon when the owner was due to arrive, but shared his thoughts on the prospect of a ban between calls on his mobile phone. "We are not sellers; we are distributors," he said. "If the government ban qat, we will find something else. It is the retailers who sell it everywhere, the Somali people, who will suffer." When the owner arrived, in a silver Mercedes, he asked us to leave.

Whether or not to ban qat has divided the Somali community. Some claim that the drug is harmless and that making it illegal would deny them a harmless pastime. But others welcomed the government ban. Abukhar Awale, a long-time campaigner against qat who calls himself a former addict, said the drug was tearing the community apart.

"For the Somali community, qat is the biggest barrier to our integration. It's segregating Somali youngsters from wider society. They are in the marfashes, They do not contribute, they don't speak English, they don't feel they are part of the society," he said. "If they ban it, we will have the biggest party in the park."

Others in the community agree. Mohamed Ibrahim, the chair of the London Somali Youth Forum, wrote to the home secretary this week to plead for the ban. "Over the past five years as a youth activist and a local government civil servant, I have witnessed the direct and unintended consequences of qat use in the UK," he said.

Those who wanted the ban claim that qat takes men away from families, by luring them to marfashes where some will chew on the plant from afternoon to early morning. They are not there when their children come home from school, nor awake in the morning to take them there.

Scientists on the Advisory Council on the Misuse of Drugs (ACMD) admit that there are health problems linked to qat use. But Tim Williams, a consultant addiction psychiatrist in Bristol, and a member of the ACMD group that produced a report on the herb in 2005, said all the social problems facing the Somali community cannot be blamed on qat.

Williams pointed to the Yemeni community in Britain, which has been using qat for generations. "If it was qat that caused all these problems, we would see it in the Yemeni community. We can be very clear it's not qat causing those problems.

"It's much more about the social problems this particular community faces. You have young men coming out of a chaotic, conflicted environment, coming to the UK to seek refuge, and they are incredibly disadvantaged. Qat chewing is being demonised by certain people in the community to explain away the problems they are suffering."

The medical review by the ACMD found that some heavy users who were vulnerable developed psychosis, and there is some hint that too much can be toxic to the liver. But there is no sign of more liver damage among the Yemeni community, Williams said, so the jury is out.

The argument for a ban has to be weighed against the negative consequences, he said. "From the US and Swedish experience we know that if we ban it legitimate businessman don't get involved and that qat distribution and supply turns into the hands of the usual criminal gangs who tend to be involved in the drug trade.

"So qat may be sold with cannabis, or maybe cocaine. So if you ban it, you protect a small population who may be harmed by using qat, but you are creating a greater level of harm to the rest of the community, who you will be criminalising, and exposing to more dangerous substances."

The UK ban was motivated in part by a wish to prevent Britain becoming a hub for illegal exports of qat to other countries. Most of northern Europe and the US and Canada have banned the plant, which must be eaten within 48 hours of being picked to be at its best. But Williams said the ban was "incredibly disappointing" and it was naive for the US and others to think illegal imports of qat would fall with a ban in Britain.

"Just because the UK bans it doesn't mean they won't continue to get it from elsewhere. If there's a profit margin it will happen," Williams said.