Six British military policemen were killed and eight other soldiers wounded in two incidents in southern Iraq yesterday morning.
It was the biggest setback to British forces since the war was declared officially over.
The bodies of the dead - the largest number of British military personnel to be killed by hostile fire since the war began and the first to die since April 6th - were recovered from the town of al-Majar al-Kabir.
Although US forces have come under daily attacks in Baghdad and central Iraq, British forces in control of Basra and other southern areas had not encountered any major violence since Saddam's fall.
In an emergency statement last night, the UK Defence Secretary, Mr Geoff Hoon, told the Commons that the dead soldiers had been training the local Iraqi police.
He said local information suggested they may have been involved in what he described as "an incident" at the police station.
"I regret that at this stage I am unable to provide any further details. British commanders are obviously investigating the situation as a matter of urgency," Mr Hoon said.
In a separate incident at 7.30 a.m., troops from the 1st Battalion of the Parachute Regiment engaged in a routine patrol in the same town were ambushed by guerrillas.
Their two vehicles were attacked with rocket-propelled grenades, heavy machine-guns and rifle fire from "a large number of Iraqi gunmen", the Defence Secretary told MPs.
The paratroopers returned fire and called for assistance. When a troop of Scimitar vehicles and an RAF helicopter arrived, they also came under fire. Mr Hoon said eight British troops were injured - one on the ground and seven in the helicopter - and were taken to a field hospital.