ARMY investigators were last night racing to unravel the mystery of what happened in the moments before Iraqis turned on British soldiers - killing six Royal Military Policemen.

Service chiefs have ordered an urgent inquiry into the Army's worst casualties since the first Gulf War in 1991.

Among the dead were Lance Corporal Benjamin McGowan Hyde, 23, from Northallerton, North Yorkshire, and 21-year-old Corporal Simon Miller, from Washington, Tyneside.

A senior British officer described the deaths of the Red Caps in Al Majar al-Kabir as "unprovoked murder" and gave Iraqi authorities 48 hours to give the killers up.

But witnesses in the southern Iraq town claim the soldiers opened fire on demonstrators first.

An investigation is under way but a Ministry of Defence spokesman said it could be days before a clear picture emerged.

There were added difficulties in that there were no surviving British witnesses to the attack, he said.

"We are aware of the differing reports and we are trying to find all witnesses.

"We are having to build a picture of what happened and the whole sequence of events. People are working very hard on this and hopefully it will become clearer, but that could take a few days."

The grim picture emerged as Prime Minister Tony Blair paid tribute to the six dead men, from Colchester-based 156 Provost Company, adding they were doing an "extraordinary and heroic job".

They were killed by Iraqis in the town, near Basra, amid a reported two-hour gun battle that left a police station, where four of them were holed up, riddled with bullets.

As well as Cpl Miller and L Cpl Hyde, the other dead men are Sergeant Simon Alexander Hamilton-Jewell, 41, from Chessington, Surrey; Corporal Russell Aston, 30, from Swadlincote, Derbyshire; Corporal Paul Graham Long, 24, from Colchester; and Lance Corporal Thomas Richard Keys, 20, from North Wales.

In Majar al-Kabir, townspeople have been given 48 hours to hand over the gunmen responsible while Defence Secretary Geoff Hoon ponders the possibility of sending thousands more troops to Iraq.

It depended on the results of an urgent review of numbers and tactics, he said.

The British military spokesman in Iraq, Lieutenant-Colonel Ronnie McCourt, said the attack was unprovoked, adding: "It was murder."

But local policeman Abbas Faddhel said the British soldiers shot dead four civilians during a protest against heavy-handed British tactics during a weapons sweep.

One witness said British soldiers first fired rubber bullets and then live ammunition into the crowd.

Faddhel said two Britons were killed at the scene and the other four at a nearby police station, where they had been chased and besieged.

One British soldier was shot dead in the doorway of the building and the other three were killed after Iraqi gunmen stormed the station, said Salam Mohammed, a 30-year-old member of a municipal security force.

Another British military spokesman, Captain Adam Marchant-Wincott, could not say whether the British forces had fired at the demonstrators but stressed they would have done so only if their lives had been threatened.

The clash raised fears that violence is spreading to formerly calm regions of Iraq, despite assurances by US officials that they were mopping up resistance.

Majar al-Kabir is a mostly Shi'ite town about 180 miles south-east of Baghdad and just south of Amarah.

The British are helping train local Iraqi police there.

The gun battle was the second attack on British troops on Tuesday after eight soldiers from 1st Battalion, the Parachute Regiment, were wounded, three of them seriously, in a fire fight with Iraqi gunmen in the south of the country.

Forty-three British troops have died - 19 in accidents - since the war began.