The Taleban is believed to be stockpiling vast quantities of opium after a bumper crop last year sent the price of the drug spiralling downwards.

The Taleban, which relies on sales of the drug for arms purchases, is hoping to hold onto its stock for long enough for the value to rise again, officials in Kabul said today.

A Western counter-narcotics official said opium from the harvested poppy crops was currently valued “as low as $50 a kilo”.

Antonio Maria Costa, the head of the United Nations Office on Drugs and Crime (UNODC) said: “Last year Afghanistan produced about 8,000 tonnes of opium. The world in the last few years has consumed about 4,000 tonnes, so this leaves a surplus. It is stored somewhere and it’s not with the farmers.”

Mr Costa told BBC’s “File on 4” programme that the Taleban still managed to make about $100 million (£50 million) last year from the opium business, “charging poppy farmers ten per cent tax for their harvested crops".

Last year was a near-record poppy crop. This year’s harvest which finished in May is expected to produce a lower yield, although the final figure will not be published by UNODC until August.

One counter-narcotics specialist in Afghanistan said it took time to assess satellite imagery to distinguish between wheat crops and the poppy fields. “But partly because of a bad winter for poppy-growing and steps taken by individual governors to stop poppy cultivation, the yield for 2008 is likely to be lower,” the specialist said.

“If you look at the poppy fields, the bulbs [from which the opium resin is extracted] were the size of Brussels sprouts this year. Last year they were as big as tennis balls,” the specialist said.

The stored opium from last year’s above-average cultivation would be heavily protected, officials said. “The one thing the Taleban and the war lords controlling the opium will not be afraid of is arrest by the police, so they will just sit on the opium which can be kept for two years,” one counter-narcotics official said. The Taleban are known to take about one tenth of the opium harvested from the poppies.

Mr Costa also said that the insurgents made extra money from protecting the opium laboratories and providing armed escorts for the lorries carrying the opium over the Afghan border into Pakistan.

One counter-narcotics official in Kabul said that poppy-planting in Helmand province, where the British troops are based, and also in neighbouring Kandahar, had been slightly down this year. But in other areas, especially in the north and east, there had been dramatic falls in poppy-planting. “Thirteen out of the 32 provinces in Afghanistan now don’t grow poppies,” the official said.

However, although the opium yield is expected to be lower this year than last year, officials said it was more realistic to compare the 2008 crop with 2006 because the 2007 production was exceptionally high.

Nato today welcomed the announcement from Berlin that 1,000 more German soldiers are to be sent to northern Afghanistan, boosting their troop numbers to about 4,500.

Franz Josef Jung, the German Defence Minister, said the extra troops would be involved in reconstruction efforts. He said he would be asking parliament to extend the mandate for the deployment in Afghanistan to December next year. The German troops are mostly stationed in Kunduz in the north.

The possibility of sending another 1,000 troops has been under discussion for some time. “Nato welcomes the decision,” a spokesman for the alliance said.

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