By Jon Boone in Kandahar

Bombing sorties by British jets in eastern Afghanistan have destroyed what Afghan and international officials claim is the biggest narcotics haul in history.

Afghan anti-narcotics commandos made their discovery on Monday after being tipped off about the presence of what were described as "underground warehouses" close to the Pakistani border in the southern province of Kandahar.

The covered trenches, apparently built with bulldozers and earth-moving equipment, contained 236.8 tonnes of marijuana which the interior ministry said was worth $450m (€286m, £228m).

General Ab-Hadi Khalid, the deputy interior minister, said yesterday it was the "largest amount of narcotics ever to be seized in the fight against narcotics in the world".

Taliban insurgents would have stood to gain about $14m from the sale of the drugs, Nato officials said. The insurgents are thought to get as much as 40 per cent of the cash they use to bankroll their insurgency campaign by tapping into the country's booming illegal drugs -business.

Although there have been more valuable seizures of cocaine stockpiles, the discovery near the town of Spin Boldak is believed to be the largest seizure.

The quantity of hashish, which was organised into 100kg sacks, was so great that it would require 80 trucks to transport over the nearby border crossing into Pakistan. Anxious to destroy the haul before any of it could be spirited away, British Harrier jets dropped one 1,000lb bomb and two 500lb bombs on the trenches "to make them burn", a Nato spokesman said.

Afghanistan produces more than 90 per cent of the world's opium and is rapidly becoming one of the largest producers of cannabis.

"With this single find, they [the police] have seriously crippled the Taliban's ability to purchase weapons that threaten the safety and security of the Afghan people and the region," said General David McKiernan, commander of the International Security Assistance Force.

"This was the largest ever single find of narcotics in history," David Miliband, the British foreign secretary, said in a statement.

"It reflects the efforts of the Afghan government against the drug trade, and was so large that two aircraft were brought in to destroy the underground bunker in which the hashish was being stored."

In another recent operation, Afghan police seized and burned $30m of opium in Helmand province and arrested 13 drug dealers, Gen Khalid announced yesterday.

Afghanistan has been criticised for not doing enough to tackle drug producers and traffickers.

Gen Khalid said the government was "determined" to catch smugglers, and build a 5,000 strong paramilitary police force to tackle the drug networks.