Two women were among four British soldiers killed by a roadside bomb near Basra, southern Iraq, the Ministry of Defence has confirmed.
A civilian translator was also killed in the bomb blast, which targeted a patrol in a Warrior armoured vehicle.
A fifth soldier was also "very seriously injured".
Tony Blair said it was too soon to link the attack to Iran, but he accused the country of "financing, arming and supporting terrorism in Iraq".
And he contrasted the soldiers' deaths with the safe return of the 15 British sailors and marines held captive in Iran.
The British army's spokesman in Iraq said there has been a "steady increase" in the number of attacks on UK troops.
This latest incident brings the total number of UK troops killed in operations in Iraq to 140.
The women belonged to the Intelligence Corps and the Royal Army Medical Corps.
The two male soldiers who were killed alongside them were from the Royal Army Medical Corps and 2nd Battalion The Duke of Lancaster's Regiment.
The injured soldier is now being treated in the military hospital in Basra
The MoD said next of kin were being informed and no further details would be released until that process was complete.
According to the MoD, the incident happened in the early hours of Thursday as the troops returned from patrol duty.
Their vehicle first came under attack near the centre of Basra and continued on its way.
However, on a stretch of highway in the Hayaniyah district on the north-western outskirts of Basra it was hit by a massive bomb explosion which wrecked the vehicle and left a three-foot crater.
The soldiers were all inside the vehicle when they died.
After the bomb explosion British forces in the area came under attack from small arms fire and rocket-propelled grenades, and they fired back.
Iraqi police said the British forces attacked a nearby police checkpoint and detained and disarmed those manning it.
Lt Col Kevin Stratford-Wright, the British Army's spokesman in Basra, said that despite a lull last month, the number of attacks from insurgents in the region was escalating.
"There has been a pretty steady increase over recent months.
"But clearly, although they are a very small percentage of the population - probably less than 1% are hostile to us - they are capable of launching very deadly attacks as they did last night."
The BBC's Jim Muir in Baghdad said the area where the incidents took place is believed to be a hotbed of support for the radical Shi'ite militia lead by the cleric Moqtada Sadr.
Rifleman Aaron Lincoln, 18, was shot while on patrol in Basra
Two other British soldiers were also killed in Iraq this week, also while on patrol in the Basra area.
The bodies of 18-year-old Rifleman Aaron Lincoln, from the 2nd Battalion, The Rifles, and Kingsman Danny Wilson, 28, from Chindit Company, 2nd Battalion, The Duke of Lancaster's Regiment, have been flown back to RAF Lyneham in Wiltshire.
Kingsman Wilson, from Workington, Cumbria, was fatally wounded while on patrol in the al-Ashar district of Basra on 1 April. He leaves behind a wife and son.
Rifleman Aaron Lincoln, from Durham, was shot in the same area the following day.
Of those soldiers who have died in Iraq, 108 are classed as having been killed after hostile action, while 32 have died from illness, non-combat injuries or accidents, or the cause of their death is still unknown.