Two British soldiers have been killed and another injured by a roadside bomb in Amara, southern Iraq, the Ministry of Defence has said.
The three were attached to a battlegroup mainly made up of members of the Royal Scots Dragoon Guards.
Tony Blair's official spokesman said the prime minister was "sadly aware" of the deaths, which take the number of UK troops killed in Iraq to 103.
Iraqis threw stones as troops arrived at the scene after the blast.
The bombing happened as at least 50 people were killed and many more injured by at least four blasts in Baghdad - just a day after a curfew imposed following earlier violence was lifted.
A suicide attacker blew himself up after joining a queue of people trying to buy kerosene in the New Baghdad district in the east of the city.
The other blasts were thought to include a mortar attack on the National Theatre in Baghdad.
Video footage of the aftermath of the Amara blast showed British soldiers at the scene and locals throwing stones.
British troops were filmed at the scene
It also shows two Army Land Rovers, one of which was badly damaged.
The BBC's Jane Corbin she was filming in Amara when the attack happened and said the atmopshere was difficult to judge.
"I did walk along one of the main streets of Al-Amara and the situation was very hard to read, I must say. Some stones were thrown, a few people acknowledged me, but many were actively hostile."
She said the tensions in the area had ebbed and flowed over recent months.
About 8,900 UK personnel are stationed in Iraq, mainly in the south-east of the country.
"Next of kin are being informed and we cannot give further details until this process is complete and the incident has been thoroughly investigated," said Mr Blair's spokesman.
The last UK soldier to die in Iraq before Tuesday was Trooper Carl Joseph Smith, 23, from the 9th/12th Lancers, who died after a crash on the outskirts of Basra on 2 February.
Tuesday's casualties come amid heightened tensions between British troops and local Iraqis.
Two regional councils have suspended relations with British forces after a video of soldiers appearing to abuse Iraqi civilians in Amara came to light.
Last month Lance Corporal Allan Douglas, 22, from Aberdeen, was killed by small arms fire while on patrol in the city - in Maysan province north of the main British base at Basra.
On 16 July 2005 three soldiers - Second Lieutenant Richard Shearer, Private Leon Spicer and Private Phillip Hewett of C Company, The 1st Battalion Staffordshire Regiment - were killed when an explosion hit a patrol of three armoured Land Rovers in the city.