Six British military police officers have been killed and eight other servicemen wounded in two separate incidents in south-eastern Iraq.
Both incidents happened at the edge of the British area of operations within the country, in the region of the town of Amara.
They mark the heaviest losses to enemy action suffered in a single day by US-led coalition forces since the war in Iraq was declared largely over on 1 May, after the toppling of Saddam Hussein's regime.
It is also the heaviest loss of British life in a single hostile incident since UK forces entered Iraq at the start of the war in late March.
About 20 US troops have been killed in attacks in the capital Baghdad and surrounding cities and towns since President George W Bush declared that large-scale combat operations had ended.
He believes those who died have died with honour doing a very worthwhile
job, serving their country with great distinction
UK Prime Minister Tony Blair's spokesman
By contrast, British troops operating in and around the second city of Basra had until now seen no serious post-war attacks, often dispensing with their helmets and flak jackets to present a less threatening sight to local people.
British Defence Secretary Geoff Hoon said the bodies of the six Royal Military Police officers, who had been training Iraqi police officers, were discovered on Tuesday in the village of Majar al-Kabir, 25 kilometres (16 miles) south of Amara.
He told the House of Commons that the circumstances of their deaths were being investigated, but initial indications were that they were involved in an incident at the local police station.
Para patrol attacked
Mr Hoon said that a few hours earlier, two vehicles carrying troops from the 1st Battalion of the Parachute Regiment came under attack from a large number of Iraqi gunmen while on patrol in Majar al-Kabir.
The Iraqis were armed with heavy machine-guns, rocket-propelled grenades and rifles, he told MPs.
The paratroopers returned fire and called for assistance, said Mr Hoon, and a troop of Scimitar light tanks, a Chinook helicopter and extra troops were sent to the scene - and also came under fire.
The defence secretary confirmed that eight British servicemen were injured - one on the ground and seven in the helicopter - and were taken to a field hospital.
Two were subsequently transferred to a US field hospital in Kuwait for "specialist treatment for very serious injuries", he added.
"We are investigating whether there is any connection between these two incidents," Mr Hoon said in an emergency statement to the House.
"British commanders in theatre are assessing the situation and have been in contact with local leaders.
"I would certainly caution against reaching any wider conclusions about the overall security situation in southern Iraq in the UK's area of responsibility.
"Coalition forces have worked hard to secure Iraq in the aftermath of decisive combat operations. They will not be deflected from their efforts by the enemies of peace."
But Tam Daylell, an MP of the governing Labour Party, called for United Nations intervention in Iraq, warning that the "unpalatable truth" was that American and British troops were seen as an army of occupation rather than liberation.
US Defence Secretary Donald Rumsfeld has expressed sorrow over the British deaths.
He said it was a "sobering reminder" that while major combat activity was over, coalition forces were still engaged in difficult and dangerous operations.
Many Iraqis resent the presence of occupying Western troops
A US military spokesman, quoted by the Associated Press news agency, said some 25 attacks on coalition forces in Iraq had taken place on Monday and Tuesday. Most of them were believed to have been against American troops.
BBC defence correspondent Jonathan Marcus says the spectre of a long-running guerrilla war is looming as Saddam Hussein loyalists and others opposed to the US-led occupation try to pick off troops at checkpoints or in small-scale patrols.
He adds that British units had not previously come under the same level of attack, in large part because their area of responsibility in the south has a mainly Shia Muslim population which probably hated Saddam Hussein's regime even more than the occupying powers.
Tuesday also saw the United Nations special representative for Iraq,
Sergio Vieira de Mello, give voice to Iraqi concerns about the US-led occupation.
"They are indeed impatient about seeing a body emerge that is
truly Iraqi and that assumes interim executive functions for the
management of their day-to-day affairs of this country," he said.
"No foreigner can actually govern this country. Only Iraqis have
the capacity and the right to administer Iraq. But obviously the
longer it takes, the greater frustration and impatience."