A 'friendly fire' incident has claimed the lives of two UK soldiers during fierce fighting outside the southern Iraqi city of Basra.
Challenger II tanks have also seen action in Kosovo and Bosnia
The pair, serving with the Queen's Royal Lancers, were killed when their Challenger II tank came under fire early on Tuesday morning. Two others were seriously injured.
The BBC's Nick Witchell, at coalition military headquartes in Qatar, said their vehicle was hit by a shell from another British tank that either missed its intended target or rebounded off it.
The two men were named by the Ministry of Defence as Corporal Stephen John Allbutt,
35, from Stoke-on-Trent, and Trooper David Jeffrey Clarke, 19, from Littleworth,
Although the tanks are fitted with identification systems, Witchell said for the moment a technology failure was not being blamed and the deaths were being viewed as "one of the realities of war".
The families of those killed and injured have been informed.
Lieutenant Colonel Michael Riddell-Webster, commander of the Black Watch battle group, said the "friendly fire" deaths were a tragedy.
He said the commander of the Challenger tank troop involved was "clearly distraught".
"The rules of engagement, even in war, are very strict. We have trained hard
to avoid this kind of thing," he said.
"It is never done on purpose. No-one is blaming the
He said that "in the darkness and confusion, a mistake was made".
He added: "It's war and sadly sometimes
such tragedies cannot be avoided."
The incident happened while the tank unit was engaging pockets of Iraqi soldiers and militia near a bridge over the Qanat
Shatt Al Basrah canal running along the western edge of the city.
A UK military spokesman said it was during a period of multiple engagements with Iraqi forces on the outskirts of Basra.
For several days now UK forces have been fighting an estimated 1,000 Iraqi troops in the city, including members of
Saddam's Fedayeen, the Iraqi leader's most loyal soldiers.
Tuesday's fighting saw the bombardment of Iraqi mortar positions inside Basra in an effort to support what the British military says is a violent civilian uprising - denied by Baghdad - against Saddam Hussein's regime.
The two British 'friendly fire' deaths came in combat earlier in the day.
The turret of their Challenger is reported to have been blown off in the attack, which happened in pitch darkness.
An investigation has been launched.
The deaths follow an incident on Sunday when a British pilot and navigator were shot down in their RAF Tornado jet by an American patriot missile battery.
Two British soldiers have been killed during combat in the conflict so far.
On Monday a soldier from the 1st Battalion Black Watch was killed at al-Zubayr.
The first British combat victim was Sergeant Steve Roberts, from Bradford, who was believed to have been shot during rioting, also at al-Zubayr, on Sunday night.
The 33-year-old died from his injuries on Monday morning.
The number of UK armed forces personnel lost since the start of the war with Iraq now stands at 20.