Almost three-quarters of the fighting strength of the SAS is to be committed to targeting drug factories in the Taliban heartland of southern Afghanistan as part of an intensive British counter-narcotics offensive.

At least three of the four "sabre" squadrons of the Hereford-based special forces - about 200 soldiers - are to be sent to Helmand with orders to destroy the network of refineries processing raw opium resin into the heroin sold on the streets of Europe.

The SAS troopers - some being withdrawn from Iraq to take part in the operation - are to be supported by 500 paratroopers and other members of the Special Forces Support Group, a unit set up last year to add muscle to undercover missions.

The military "surge" next spring is to coincide with a new UK initiative expected to be outlined by Prime Minister Gordon Brown today to pay subsidies to Afghan farmers to encourage them to grow fruit and cereals rather than lucrative opium poppies.

A poor farmer can earn 10 times as much growing poppies than from attempting to cultivate wheat or maize in conditions that vary from searing summer drought to sub-zero winter cold.

The UN says the area under poppy cultivation in Afghanistan rose this year from 165,000 to 193,000 hectares and the harvest of opium resin increased from 6100 to 8200 tonnes. Opium production is heavily concentrated in areas of insecurity, with the British area of responsibility in Helmand now the world's biggest source of illicit narcotics.

The number of deaths in Scotland from heroin, morphine and other opium-based drugs rose to 374 in 2006, an increase of 42% on the previous year. There are an estimated 51,000 heroin addicts north of the border.

The statistics, compiled by police forces across Scotland, show nearly 71% of the deaths were attributed to heroin and morphine overdoses.

There were 1366 heroin-related deaths in England and Wales over the same period, according to coroners' data.

A military source said yesterday the special forces would target major dealers who pay Afghans to grow poppies and harvest the resin.

The UK initiative comes in response to US pressure for tougher anti-drug measures and threats to begin aerial spraying of poppy fields with defoliants. Nato officials fear such a drastic approach could drive poor farmers into the hands of the Taliban.

The SAS mounted one of the biggest raids in its 56-year history against a drug refinery in southern Afghanistan in November 2001 in Operation Trent. Some 120 troopers staged an assault, killing up to 60 drug traffickers and destroying large stocks of opium.

Nato sees eradication of the heroin trade in Afghanistan as a key plank in establishing enough physical security to allow improvements in health, transport and communications infrastructure.

Although the Taliban does not grow poppies, it exacts a tax on the product from dealers, allowing it to finance the purchase of weapons and ammunition and pay recruits.

The counter-narcotics campaign is expected to be linked to a new spring offensive by up to 8000 British conventional troops to break the Taliban hold on Helmand.

All three battalions of the Parachute Regiment, plus its Territorial support, and up to half of the Royal Regiment of Scotland's soldiers have been earmarked for deployment to Afghanistan in March.