A report from the UN office on drugs and crime recorded an overall decline in the area under opium poppy cultivation in Afghanistan from about 131,000 hectares in 2004 to 104,000 hectares in 2005.

But the document says that the figures mask massive regional differences, with opium production increasing 106 per cent in the north of the country, 98 per cent in the west and 30 per cent in the south. The report is an embarrassment to Washington and London as they claim stability and progress in Afghanistan.

"The strongest increases were in the north and west where Nato is operating," said Antonio Maria Costa, executive director of the UN office on drugs and crime. "This needs to be brought to the attention of Nato."

Two western military operations are present in Afghanistan. The US-led coalition, which entered the country after the September 11 attacks of 2001, is on a mission to eradicate the remnants of the Taliban in Operation Enduring Freedom. Since December 2003, Nato's International Security Assistance Force (ISAF) has been increasing its presence by establishing so-called Provincial Reconstruction Teams (PRT) in the north and west.

The UN document shows a 334 per cent increase in production in the region of Balkh, despite the presence of a PRT at Mazar-e-Sharif. The picture is similar in the west with a 348 per cent rise in Farah where ISAF is also present. Meanwhile officials are alarmed at the 162 per cent rise in Kandahar.

Speaking in Brussels after meetings with EU and Nato officials, Mr Costa said: "It looks like the country is dedicating some of its best agricultural land to the cultivation of opium. Is it a coincidence or is it because they feel that they are less threatened by ISAF?"

A Nato spokesman said: "We are aware of the problem and reducing the cultivation of poppy will be an effort of the international community."