Where opium poppies used to colour the plains of northern Afghanistan, towering cannabis plants now sway in the wind, filling the air with their pungent odour.

Farmers in Balkh province were banned from cultivating opium last year and have switched to another cash crop, a rich source of income that is still tolerated by the authorities.

Balkh's burgeoning hashish industry does not pay farmers quite as much as the heroin factories used to for good-quality opium. But the rich black cannabis resin produced around the northern city of Mazar-i-Sharif still pays about four times the price of cotton or wheat. It is highly prized by Afghan usersand is exported in large quantities to Pakistan and Europe.

Growing cannabis is nothing new for Afghan farmers, but the opium clampdown has transformed a minor cash crop into big business. The 2007 annual report of the UN Office on Drugs and Crime estimated a 40 per cent rise in Afghanistan's cannabis production this year from 50,000 hectares (123,550 acres) last year to 70,000 hectares this year.

The switch from opium to cannabis is the latest embarrassment to Western attempts at eradication. It also illustrates the desperation of poor farmers.

Western officials are deeply concerned that the booming drugs trade is funding the insurgency against Nato troops and driving corruption that undermines the Government in Kabul.

The UN report has found that Afghanistan now produces 93 per cent of the world's opium, mostly in the southern provinces. It highlighted poppy eradication in Balkh as a rare bright spot. But the gloss was tarnished by Balkh's cannabis problem.

This week, as gusts of snow blew in from the north, farmers were busily harvesting their plants in the flat, wintery landscape around Mazar-i-Sharif near the border with Uzbekistan.

Roadside stores keep hashish hidden among the onions and biscuits, producing thin sticks or sheets for users who drive out from Mazar-i-Sharif. “It is the best quality in Afghanistan,” one shopkeeper said with a lazy smile. “I don't keep opium any more because it is too much trouble. But hashish is good business.”

Unlike opium, cannabis is smoked by some farmers without serious social consequences. “The only thing is there seem to be more layabouts now that we grow so much cannabis.” one said.

Muhammad Qol, 44, said that nearly three quarters of his income came from cannabis. He said: “We don't smoke it, and we know it is a sin and against Islam. But my family needs the money and the Government stopped us from growing opium, so what can we do? We are saving up for a Toyota Corolla. Everyone else has a car these days. Why shouldn't poor farmers like us have one?”

Some Western officials try to look on the bright side. One said: “At least they've gone from producing hard drugs to soft drugs. It's progress, sort of.”

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