Three British soldiers have been killed and a fourth seriously wounded, after their vehicles came under attack in the southern Iraqi city of Basra.
British troops have increased security at their Basra base
The troops had been driving away from their base when, according to witnesses, gunmen in a pick-up truck opened fire and threw a grenade at them.
Those killed were all members of the Royal Military Police, the Ministry of Defence has confirmed. Their families have been informed.
Of the 10 British soldiers killed by hostile action since major combat operations were declared over on 1 May, nine have been military policemen.
Colonel Richard Barrons, the British chief of staff in Basra, told BBC News that investigations into the "brutal and determined and effective" attack were continuing.
'Sadness and determination'
The killings came after a string of attacks on British soldiers this month, and two days of rioting in Basra over power and fuel shortages.
BRITISH DEATHS IN HOSTILE ACTION SINCE 1 MAY
23 Aug: Three Royal Military Police officers die in Basra city centre ambush
14 Aug: Captain David Jones killed in bomb attack on ambulance in Basra
24 June: Six Royal Military Police officers apparently killed by angry locals while searching for weapons in Majar al-Kabir, north of Basra
Army spokesman Major Ian Poole said the mood among British troops following the attack was one of sadness mixed with determination.
"The attacks will not distract us from our mission here," he said.
The Royal Military Police described the deaths as a "tragic loss".
A statement from its corps headquarters and
training centre in Chichester, West Sussex, added: "Present in our minds though is the terrible grief which their families and
friends are now confronted with, and their colleagues, who remain committed to
bring stabilisation and peace in Iraq."
Following Saturday's attack, on one of Basra's main streets at around 0830 local time (0530 BST), soldiers sealed off the area and searched vehicles in an attempt to find those responsible.
The wounded soldier has been taken to hospital where his condition has been described as "stable".
Major Poole said the soldiers had been on a routine patrol in two vehicles - a military Land Rover and a 4X4 - when they came under fire.
He said they had been wearing body armour and that force protection measures were "constantly under review".
Colonel Barrons said: "There is absolutely no doubt in our minds that a certain amount of pre-meditation went into this. We don't think that pair of vehicles were singled out other than they were at that place at that time."
He said most of Basra's 1.5 million residents supported the British presence, but that some - including small numbers who travelled down from north Iraq - were prepared to carry out acts of terrorism.
Armed Forces Minister Adam Ingram said he was "shocked and saddened" by the deaths.
He said: "We will do all we can, working with the Iraqi police authorities, to ensure those responsible are tracked down and brought to justice."
US troops in the Sunni-dominated north of the country have so far borne the brunt of hostility against coalition control.
Major Charles Heyman, editor of Jane's World Armies, said: "I'm afraid today's attack was just a matter of time. Over the last two weeks we have seen a
good indication of what is to come."
Former Army officer and defence expert Michael Yardley said a terror campaign against occupying forces had always been expected.
He said: "We know that Saddam Hussein planned for this contingency - to bring chaos and resist unconventionally."
Paul Bremer, the top US civil administrator in Iraq, insisted that those seeking to hinder efforts to bring stability to Iraq would fail.
In a separate incident, an official from the British embassy in Baghdad announced that staff there had been evacuated on Wednesday.
It followed what he called a credible threat of a bomb attack such as that which stuck the UN building last week.