Afghanistan, the world's biggest opium producer, is set for another bumper crop this year, providing a windfall for the Taliban who tax farmers to finance their fight against government and foreign forces.

More than six years after US-led and Afghan forces toppled the Taliban, the failure to bring opium production under control means Afghanistan is now locked in a vicious circle. Drug money fuels the Taliban insurgency and corruption, weakening government control over large parts of the country, which in turn allows more opium to be produced.

The United Nations Office on Drugs and Crime (UNODC) predicted the 2008 opium crop would be similar to, or slightly lower than, last year's record harvest. In 2007, Afghanistan had more land growing drugs than Colombia, Bolivia and Peru combined.

"While it is encouraging that the dramatic increases of the past few years seem to be levelling off, the total amount of opium being harvested remains shockingly high," said UNODC chief Antonio Maria Costa.

Opium is processed into heroin and smuggled mainly to Europe, where users often turn to crime to pay for the drug. "Europe and other major heroin markets should brace themselves for the health and security consequences," he said.

Opium poppy cultivation is concentrated in the south where the Taliban are strongest, and where British troops are based. Opium production is growing "at an alarming rate" in the south and west, the UN said.

All poppy farmers surveyed in southern Afghanistan said they paid 10% of their opium income to the Taliban or corrupt government officials.

"This is a windfall for anti-government forces," Costa said. "Further evidence of the dangerous link between opium and insurgency."

The report comes as Afghan ministers and international donors meet in Japan to discuss developments in Afghanistan. Britain is pushing for long-term investment in infrastructure and assistance for Afghan farmers.

Afghanistan is calling for more aid to stamp out opium production, but diplomats and analysts say President Hamid Karzai has failed to deal with corruption among government officials.

"We can only fight drugs in Afghanistan by the support of the international community," said General Khodaidad, acting Minister of Counter Narcotics, who uses one name.

Japan, host of the two-day meeting, responded by announcing new assistance of £55m, including £5m for police reform and £4.5m for border management.

However, Afghanistan's plea for more assistance comes as the United States and its allies struggle to co-ordinate policy in the face of rising Taliban attacks. Canada has threatened to pull out unless other Nato countries contribute more.

Violence in Afghanistan worsened last year and at least 10,000 people, including about 300 foreign troops, have been killed in Afghanistan in the past two years, aid groups say.

Meanwhile, the number of Afghan civilians killed by accident by US or Nato forces in air strikes and ground battles doubled between 2006 and 2007, the aid agencies claimed.

Human Rights Watch and the Project for Defence Alternatives (PDA) said that 272 men, women and children died in bombing raids, 62 were killed by ground fire and 16 were lost to a combination of the two last year.

This contrasted with 116 known deaths from coalition bombs and 114 from small arms and artillery fire in 2006. The coalition's increasing reliance on air power to compensate for the lack of troop numbers on the ground was the major cause of the increase in "collateral damage", said a PDA spokesman.