A suicide attack in Iraq has left three Britons dead, amid a surge of violence in the run-up to the January elections.
US soldiers secure the green zone checkpoint after car bombing
Two of the victims worked for US security company Kroll, while the third worked for one of its clients, business consulting company BearingPoint.
A fourth person, an American civilian, also died in the attack.
In a written statement, Kroll said the men had been killed in a suicide attack on one of its vehicles during a routine movement of personnel.
A convoy of several vehicles had just left the heavily guarded "green zone" in the heart of Baghdad, which houses the US embassy and government offices.
"We are very proud of the professionalism and dedication of our late comrades," the statement added, describing them as "extremely brave and loyal men" who would be greatly missed by their colleagues.
"Without the devotion to duty of care to our clients, as displayed by people such as these, the reconstruction effort would be well nigh impossible," the statement said.
Armed guards working for security firms in Iraq earn up to £1,000 a day.
The Foreign Office confirmed that three Britons had been killed but would not give any more information until next-of-kin have been informed.
There were no reports of any of the US troops manning the checkpoints being hurt in the blast.
There are currently no exact figures on how many Britons are working as security personnel in Iraq, although about 200 are believed to be working for the government.
Many more are working for various security companies, although the government does not have an overview of their numbers.
"An American company that has been contracted by, say, US Aid to go and rebuild a school, will be responsible for subcontracting their own security," a spokesman for the Foreign Office said.
"It happens that the best security companies in the world are British, so a lot of them come to Britain to look for their security, but they don't go to the British government - they contact the companies directly," he added.
"I've heard that there's about 20,000 security personnel only in Baghdad, and the vast majority of them are British."
The bombing formed part of a wave of violence in Iraq which seems to have been gathering momentum with the approach of controversial elections scheduled for 13 January, the BBC's Jim Muir said.
It came on a day when at least three other suicide car bombs and a number of other attacks claimed at least 30 lives throughout the country.