Everyone gets a cut but some more than others. The RUF in Sierra Leone sells its diamonds to dealers, predominantly Lebanese, for 90 to 95 per cent of their rough value. The dealers smuggle them across Sierra Leone's porous borders into Liberia, and to a lesser extent, Guinea.

The dealers then arrange for the stones to be despatched mainly by air to the main trading centres for rough diamonds such as Antwerp and Tel Aviv.

Couriers typically receive a commission worth one per cent of the value of the stones. Some buyers use secret Swiss bank accounts - called "mountain dollars" in the trade - to cover up their transactions.

An Antwerp diamond dealer said on occasions the dealers smuggling the stones into Liberia could sell them for up to 120 per cent of their value because drug runners in Monrovia were keen to launder money.

The dealer also recounted how, en route from Sierra Leone to Belgium, he stopped off in Conakry in Guinea. Customs and police officers whisked him through passport control in the hope of persuading him to pay a bribe in return for allowing him to smuggle out diamonds. "They rolled out the red carpet," he said.

Antwerp, the ancient Flemish town in Belgium, is the biggest trading centre for rough diamonds with a turnover of around $23bn a year. It is a natural destination for the smuggled stones. The city's diamond district is concentrated in three dingy streets near the railway station. It is a small, tightly-knit world dominated by Indians, Belgian and Israeli Jews and the Lebanese.

The industry is still run on the old-fashioned rule that business relationships can be based on trust rather than formal contracts. But this climate of secrecy lends itself to abuse.

An Antwerp dealer estimated that rough diamonds worth $3bn are smuggled into Belgium every year. Other dealers said the figure would be much lower, but many admitted there was "trading in black" - slang for black market trading.

A British government official said one of the authors of the Fowler report had privately disclosed that "99 per cent of Unita rough diamonds were being traded in Antwerp. Nobody declaring them to Customs, these were smuggled in".

Another Antwerp dealer blamed indifference in the trade to conflict diamonds. He said: "If you go to the diamond dealer's office and say 'I want to buy these', you do not ask where they come from. If the price is right and you think you can make money on the parcel, you buy it."

Peter Meeus, general director of Antwerp's diamond high council, which co-regulates the industry with the Belgian government, said the black market in diamonds had been "marginalised".

He insisted Unita diamonds were no longer available in Antwerp. "We would hear. It's a small world, it's only three streets. It's 5,000 people, and let's say 500 important people who all know each other. It would be known."

Meanwhile non-governmental organisations such as Global Witness are determined to raise public awareness about conflict diamonds. The main battle will be fought in the US, where annual sales of diamond jewellery represent almost half of the $55bn sold worldwide.

The plush showroom of Tiffany & Co, on New York's 5th Avenue, is a world away from the diamond pits of Kono. And yet, some of the diamonds glittering in the store's windows could have been discovered by the mud-caked hands of the Sierra Leonean diggers.

Asked by a would-be groom about the origin of the diamonds, the Tiffany's salesman could not guarantee they did not come from conflict zones in Africa.

"Tell the bride not to concern herself too much or she won't wear a diamond ring," said the salesman.

Additional reporting by David Buchan, Neil Buckley, Thomas Catan, Michael Holman, Carola Hoyos, Khozem Merchant, Sathnam Sanghera and Mark Turner