Iran has been training Iraqi militiamen to use the roadside bombs believed to have killed more than a dozen British soldiers in the past two years, US intelligence sources confirmed yesterday.

The Iranian Revolutionary Guard Corps (RGC) has drafted in combat-experienced instructors from the Lebanese Hizbollah guerrilla movement to train Shi'ite insurgents operating in the UK sectors of Maysan province and Basra.

Ahmad Abu Sajad al Gharawi, a former Mahdi Army militia commander, was named yesterday as the leader of the main Iran-trained group in Maysan.

He is believed to have been responsible for the attack that killed Lance-Corporal Alan Brackenbury, whose death in 2005 near Al Amarah gave coalition investigators their first direct links between the lethal new booby-traps and Iran.

As revealed by The Herald yesterday, forensic evidence from the ambush site showed that an explosively-formed projectile (EFP) - a copper slug capable of penetrating the armour of a 70-tonne US Abrams tank - had been used to destroy the British Land Rover. Military-grade P4 plastic explosive traceable to Iran was also found.

Military engineers who examined the equipment said it was identical to bombs supplied by the RGC to Hizbollah militiamen fighting Israeli troops in Lebanon. Radio receivers used as back-up detonators were set to Iranian military frequencies.

Al Gharawi, who is now on the Iraqi government's most-wanted list, replaced Ahmad Jawwad al Fartusi, another Mahdi splinter group commander known to have imported EFPs for use against coalition troops before his arrest by British forces in September, 2005.

An intelligence source said yesterday: "We have known for a long time that the RGC has been stirring up trouble among the Shi'ite militias. What we lacked was - almost literally - a smoking gun to tie them to the active insurgency.

"That's now coming together, piece by piece, starting with the killing of L-Cpl Brackenbury. The use of Hizbollah as a proxy advisory group also ties the RGC to the insurgency.

"Hizbollah is bankrolled, trained and armed by the RGC and does not make a move without that group's permission."