Six British military personnel have been killed and eight wounded in two separate incidents in southern Iraq, the UK prime minister's office has said.
Downing Street said both incidents happened near the town of Amara, 200 kilometres (125 miles) north-west of Basra.
These are by far the heaviest casualties suffered by British forces - who control a wide swathe of southern Iraq - since the war was declared over on 1 May after the toppling of Saddam Hussein's regime.
Dozens of US troops have been killed in attacks and accidents in the capital Baghdad and surrounding cities and towns since that date.
Details of the first incident, in which the six people died, have not yet been disclosed.
Prime Minister Tony Blair's office said families of the soldiers killed were still being informed.
The UK Ministry of Defence said that in the second incident, troops from the 1st Battalion of the Parachute Regiment came under fire while on patrol south of Amara.
One soldier was wounded and two vehicles were destroyed, the MoD said in a statement.
"In responding to the incident, an RAF Chinook helicopter carrying a Quick Reaction Force came under fire as it landed," the statement said.
"Seven personnel on board the helicopter were wounded, three of them seriously.
"All were extracted by helicopter and are receiving treatment.
"We are investigating the incidents, including whether or not they are related."
There were separate reports on Tuesday of an attack on US troops in Ramadi, to the west of Baghdad.
Up to five Iraqis are reported to have been killed when US forces retaliated after coming under fire. Two Americans are said to have been injured.
A US military spokesman, quoted by the Associated Press news agency, said some 25 attacks on American-led coalition forces in Iraq had taken place on Monday and Tuesday.
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 who are exposed 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 Hussen's regime even more than the occupying powers.
The 1st Battalion of the UK's Parachute Regiment has about 650 soldiers in Iraq who operated mainly around Basra and the southern oilfields during the conflict.
One of their tasks since the end of the war has been to look after the British embassy compound in the Iraqi capital, Baghdad.
The soldiers, who are based at Connaught Barracks in Dover, went to Iraq in February and are due to start returning to the UK from 30 June
well its finally happened. firstly condolences to their families, secondly id like Bliar to look their loved ones in the eyes and tell them what exactly they died for
