By Kate Clark
BBC News, Afghanistan
The Taleban 'taxes' poppy farmers
The Taleban made an estimated $100m (£50m) in 2007 from Afghan farmers growing poppy for the opium trade, the United Nations says.
Antonio Maria Costa, head of the UN's Office on Drugs and Crime (UNODC), said the money was raised by a 10% tax on farmers in Taleban-controlled areas.
The UN estimates last year's poppy harvest was worth $1bn (£500m).
Mr Costa said the Taleban made even more money from other activities related to the opium trade.
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"One is protection to laboratories and the other is that the insurgents offer protection to cargo, moving opium across the border," Mr Costa told the BBC's File on 4 programme.
The final figures for this year's harvest have yet to be released but yield and proceeds are likely to be down due to drought, infestation and a poppy ban enforced in the north and east of Afghanistan.
This would lower revenue, "but not enormously", Mr Costa said.
The past few years have seen abundant yields from poppy farming, with Afghan farmers cultivating more than the global demand.
"Last year Afghanistan produced about 8,000 tonnes of opium," Mr Costa said.
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"The world in the past few years has consumed about 4,000 tonnes in opium, this leaves a surplus.
"It is stored somewhere and not with the farmers," he added.
The stockpiles represent hundreds of millions of dollars and it is not known whether they are possessed by traffickers, corrupt Afghan officials and politicians or the Taleban.
British officials say that drugs money funds the Taleban's military operations.
"The closer we look at it, the closer we see the insurgents [are] to the drugs trade," said David Belgrove, head of counter narcotics at the British embassy in Kabul.
"We can say that a lot of their arms and ammunition are being funded directly by the drugs trade."