The soldier, from the 7th Armoured Brigade, died as a result of injuries sustained from an explosion. Three other soldiers were injured, one seriously.
The attack took place in Um Qasr, in Basra Province, at around 8.34am local time, the MoD spokeswoman said.
The MoD spokeswoman said: "We can confirm that today a British soldier from the 7th Armoured Brigade died from his injuries sustained as a result of an explosion at approximately 8.34am local time at Um Qasr, Basra Province.
"Three other soldiers were injured, one seriously, in the same incident and are receiving medical treatment at Shaibah medical facility.
"The incident is under investigation."
The MoD spokeswoman said further details would be released once next of kin had been informed.
The latest death brings the number of British troops who have died since hostilities began almost three years ago in Iraq into triple figures for the first time.
Only yesterday, a soldier from the 7th Armoured Brigade, serving with the 1st Battalion The Highlanders, was killed by enemy fire in southern Iraq.
Lance Corporal Alan Douglas, 22, was on his first tour of duty in the region and had been in the country for just seven weeks, according to the Daily Mirror.
He was shot dead after coming under small-arms fire on patrol in the volatile Maysan Province.
Of the 100 servicemen and women who have died while on Operation Telic, 77 are classed as being killed in action, while 23 died from illness, non-combat injuries, accident or an unknown cause.
In addition, 230 British troops have been injured in enemy action since the March 2003 invasion.
After yesterday's shooting, L/Cpl Douglas's father Walter, from Aberdeen, told the Daily Mirror that his son had not wanted to go to Iraq.
"He was against the war. He couldn't see the point of it. The lives of 99 young men have now been lost - and all for nothing," he said.
Mother Diane added: "He was home just before Christmas. That's the last time we saw him.
"He told me he wasn't looking forward to Iraq. He knew himself it wasn't going to be a good one."
L/Cpl Douglas had seen service in Bosnia and Kosovo, and trained in Canada before heading to Iraq, the family revealed.
The Conservatives' former foreign secretary Sir Malcolm Rifkind - who opposed the war - said he was "deeply saddened".
But he insisted that it was not yet the time for a wholesale withdrawal of British forces.
"We cannot just cut and run. The British and American governments have helped create these problems. For as long as our presence is helping to improve the situation, we have to see it through."
He said he hoped the election of a democratic government would help enable a "significant" reduction in troop numbers this year.Reuse content