Four British servicemen have been killed in an attack on a patrol boat in southern Iraq.
Another three were seriously injured in the attack, which took place during a routine patrol along the Shatt al-Arab waterway in Basra.
The boat was hit by an "improvised explosive device", said Captain Tane Dunlop, speaking from Basra.
Defence Secretary Des Browne paid tribute. It brings the total number of UK troops killed in Iraq to 125.
Capt Dunlop said the injured servicemen had been taken to a field hospital where they were undergoing surgery.
At least one was suffering from potentially life-threatening injuries, he said.
An investigation was under way involving weapons specialists and an extensive information-gathering network across the city, he added.
The Foreign Office said the families of those killed were being informed and it was too early to release details of the servicemen's units or ranks.
The attack took place at 0950 GMT (1250 local time) on Sunday and is thought to be the first such attack on a patrol boat carrying British personnel.
On the same day, two suicide bombers killed 35 people and wounded 60 at a police commando recruiting centre in western Baghdad, police said.
And the US military said three American soldiers were killed in combat in western Al-Anbar province.
British military personnel have been patrolling the Shatt al-Arab waterway, which borders Iran, since 2003.
It was thought to be a safer way to travel between bases than going by road.
Captain Dunlop said: "The use of improvised explosive devices is very common in Iraq, more often further north but there have been a number of such attacks in the Basra province and Basra city.
"It is slightly unusual in that this time it was targeting a boat, but they're usually used against any form of multi-national forces or personnel."
The deaths came on the same weekend that troops in Basra and elsewhere in Iraq paid their respects to lost colleagues, to mark Remembrance Day.
Defence Secretary Des Browne said in a statement that his thoughts were with the families of those killed.
"Today at the Cenotaph and across the country, we remembered those brave souls that have served this nation across the years," he said.
Basra, Iraq's second city, is located on the Shatt Al-Arab waterway
"That dedication is still evident and the events today in Iraq are a stark reminder of the perils they face."
The Iraqi ambassador to the UK, Salah Al-Shaikhly, offered condolences to the families of the dead and injured.
He added that the role of British forces in Iraq was "critical", saying: "Our message to the people of Britain is that we are deeply grateful for their effort to help us build our young democracy.
"We Iraqis share with them the sorrow for the sacrifices we are making daily at the hands of perpetrators of terror and violence."
Liam Fox, the shadow defence secretary and Sir Menzies Campbell, leader of the Liberal Democrats, expressed their sympathy to the families of those killed.