Afghanistan, already the world's top opium supplier, is now the world's biggest producer of cannabis, according to United Nations drug experts.

There is large-scale cultivation of the drug in half of the country, resulting in 3,500 tons of hashish worth an estimated £61m annually, according to the first assessment of cannabis in Afghanistan by the UN Office on Drugs and Crime. It warns that the threat from the drug needs to be dealt with to deny the Taliban the millions they make in protection taxes paid by farmers and drug smugglers.

The focus on opium has resulted in cannabis being overlooked, according to Antonio Maria Costa, executive director of UNODC. "A concentration of cultivation in the southern part of Afghanistan shows that the Taliban and those insurgents that control the southern parts of the country are not only funding themselves by trafficking opium but also by trafficking cannabis. It's the same area."

Sporadic government crackdowns have merely resulted in a shift in production to the south of the country, with most cannabis now grown in regions where the insurgency is at its strongest. There are close links with the heroin trade, with more than 67 per cent of cannabis farmers also growing opium, according to the report, which is based on a survey of more than 1,600 villages across the country. UN experts estimate that 60,000 households are now growing cannabis.

Although cheaper than opium, cannabis can potentially earn a farmer more because it yields twice the quantity of drug per hectare and is cheaper and less labour-intensive to cultivate. Farmers can earn £2,173 per hectare – more than the £1,304 they get from opium. But since cannabis has a short shelf life and is grown only in the summer, when there is less water available, they continue to cultivate opium as their main drug crop.

A British-backed paramilitary unit is hunting down Afghan drug lords as part of a new strategy against the drug trade. Four helicopters have already been provided by Britain for airborne assault missions by the Afghan Special Narcotics Force, with another two being made available to an Air Interdiction Unit this year. Targets are being drawn up by an Interagency Operations Co-ordination Centre in Kabul, led by officers from Britain's Serious Organised Crime Agency and the US Drug Enforcement Administration. But efforts are hampered by high levels of corruption and drug use among police, with 68 per cent testing positive for cannabis.