"You'd be stupid not to grow opium," he said, gesturing at the healthy lettuce-like plants pushing out of the cracked earth. "If the Americans give us some money, we'll stop planting poppy. If they don't, we'll carry on."

Mr Rahman lives in Singesar, a dusty village of terraced vineyards and pomegranate trees half an hour's drive from the southern desert city of Kandahar.

The village is famous because Mullah Mohammad Omar, the Taliban's fugitive leader, used to live here - a fact that gives Mr Rahman's flourishing opium garden extra piquancy.

Two years ago Mullah Omar issued an edict outlawing opium production across Afghanistan, which was at the time the world's largest producer of heroin. Taliban soldiers armed with Kalashnikovs ruthlessly enforced the decree.

"I grew tomatoes and other garden vegetables last year," Mr Rahman said. "Before that the Taliban let us plant poppy."

Nobody quite knows whether Mullah Omar's edict was inspired by Islamic principle, was a cynical trick to drive up the price or a last-ditch attempt to appease the international community.

Since the mid-1990s the Taliban had earned millions of dollars from the heroin trade.

Either way, UN officials last month confirmed that poppy production fell in 2001 in Afghanistan by 91% - from 82,172 hectares to 7,606, with most of that grown in areas controlled by the Northern Alliance.

But with the Taliban's demise, farmers across Afghanistan have reverted to their old, lucrative ways. America's bombing campaign has had a result not anticipated by Pentagon strategists - everyone is planting opium again.

"I can make $1,600 (£1,150) from this small poppy patch here," Mr Rahman said, pointing to his modest kitchen plot.

"If I sell all of the grapes over there, I'll only make a fraction of that," he added, gesturing towards a giant, rolling vineyard framed by low mountains and morning sunshine.

According to opium farmer Abdul Ali, the harvest season between May and July is a happy time in Singesar. "We all collect the poppy resin together, including the children. Even women do it because the crop grows very high and nobody can see their faces. We are glad of the money."

The eradication of opium is one of the first big tests for Hamid Karzai, the pro-American leader of Afghanistan's new interim authority. Mr Karzai has taken an uncompromising line on drugs, and called for all poppy production to stop.

But his control over much of the country is tenuous; his fledgling administration lacks resources and his local officials fail to inspire the same kind of dread that the Taliban did. UN officials privately concede that Afghanistan is heading for a bumper opium crop this year, with much of it destined for Britain and Europe.

One senior UN official based in Kandahar said: "The Taliban ban was implemented almost 100% Already we know farmers are planting opium again. Without proper enforcement, advocacy, and assistance from the donor community, the problem won't go away."

Mr Karzai's representatives are - on the surface at least - doing their bit. Three days ago Kandahar's new governor, Gul Agha, closed down the city's opium bazaar, a venerable city institution that had survived last year's poppy ban.

"There is nothing left for us now but to sit and drink tea," Shau Ali, a 35-year-old opium trader lamented, sitting on the carpet of his empty bazaar shack, decorated with glossy pictures of the Gulf.

"We are very sad because we don't have a job anymore. We are trying to persuade the government to let us sell off our remaining stocks."

Mr Ali said a kilo of opium currently cost between $2,200-$2,700 (£1,550-£1,900), down from last year's price of $3,300 (£2,350), when there was no prospect of a fresh crop. But nobody at the opium bazaar seemed genuinely miserable: the business had, it appeared, merely shifted from the front of the shop to a small back room accessible via a waist-high door.

Back in Singesar, the local security chief revealed that Gul Agha had instructed him not to worry too much about digging up this year's poppy harvest - a move that would undoubtedly heap much unpopularity on the new governor's head.

"There's not much we can do this year because the poppy has already been planted," Agha Wali said. "We'll make a start next year."

With the Taliban gone, ending Afghanistan's status as the world's largest heroin producer is clearly going to be an uphill task. In the last year before the ban came into effect the trade was worth $98m (£69m) to Afghanistan's farmers, with most of the buyers wealthy businessmen from Iran and Pakistan.

Opium has flourished in the country's southern desert region - as well as in northern provinces like Badakshan - since the time of Alexander the Great. Unlike wheat, it requires little water and is ideally suited to the country's arid valleys and unreliable rivers.

Opium grew in Afghanistan during the time of King Zahir Shah - who returns from exile next month - as well as throughout the Russian invasion, and the turbulent mojahedin years. Few believe Mr Karzai can wipe it out.

His predecessor Mullah Omar, who was Singesar's mullah before founding the Taliban in 1994, did not grow it himself, locals revealed. "During the Taliban time there was good law and order because there was one government," one villager, Mirwais, recalled. "Now there are lots of different governments and everyone wants to be the boss."

Mullah Omar returned rarely to Singesar after becoming the Taliban's leader - only at night and in a fleet of Toyota pick-up trucks with darkened windows. His last visit was on October 12 - five days after the American bombing raids against Afghanistan began. A US missile struck his car, killing his 10-year-old son, although he escaped.

Soon after this, Mr Rahman and his fellow farmers dug up their fields and started planting opium again. The cress-like shoots can already be seen growing close to Mullah Omar's old house, a ghostly, deserted, two-room building of brick-mud walls. Were he ever to return, Mullah Omar would probably approve.

Bitter harvest

· Afghanistan is the source of 75% of the world's heroin and 90% of the supply to the UK.

· Afghanistan's heroin production increased by 100% between 1988 and 1991 to 2,000 tonnes and then expanded to the bumper harvest of 4,600 tonnes in 1999.

· Poppy production fell in 2001 in Afghanistan by 91% - from 82,172 hectares to just 7,606, with most of that grown in areas controlled by the Northern Alliance.

· Drugscope believes there are 227,000 regular heroin users in the UK. They consume 30 tonnes of heroin a year