British soldiers have been in the Basra province since 2003
Corporal Gordon Alexander Pritchard, 31, a soldier from the Royal Scots Dragoon Guards died in a blast in Umm Qasr, Basra province.
His parents Jenny and Bill said in a statement he was "the epitome of a modern, professional soldier" and "extremely proud" of his regiment.
It follows the death of L/Cpl Allan Douglas, 22, who died on Monday.
The statement by Cpl Pritchard's parents said he had followed family tradition by serving with the Royal Scots Dragoon Guards.
"He was a well-trained, well-motivated soldier serving in a regiment that he was extremely proud of, as did his father and elder brother," it said.
"He was a loving son, and a very proud family man, and he will be deeply missed by us all."
Prime Minister Tony Blair said it was a "tragedy" but British troops would remain in Iraq for as long as needed.
Three other soldiers were injured, one seriously, in the Umm Qasr blast, which took place at 0834 local time (0534 GMT).
The MoD said all three were receiving treatment at the Shaibah medical facility and the explosion was being investigated.
Rations run
Cpl Pritchard, who was married with children, was commanding the lead Land Rover in a three-vehicle convoy. The MoD said the convoy was on a routine rations and water run when the explosion was detonated.
An MOD spokesman expressed "our deepest sympathy" to Cpl Pritchard's relatives and friends "at this sad time".
Umm Qasr, Iraq's biggest port, was one of the first cities to be captured by British forces in May 2003.
Since then, it has largely avoided the kind of violence seen in Baghdad and, to a lesser extent, the city of Basra.
Defence Secretary John Reid said it was an appropriate time to reflect on the "dedication, courage, professionalism and sacrifice" of the armed forces and their families.
UK CASUALTIES IN IRAQ 100 service personnel killed 77 died in action 23 from non-combat injuries
UK deaths month-by-month
The family of L/Cpl Douglas, who was shot and killed in Maysan province on Monday morning, have spoken about his death.
His mother said her son had been back to his Aberdeen home over Christmas and had not wanted to return to Iraq.
Diane Douglas told the BBC it was a "damn disgrace" that young people were being killed in Iraq, adding: "I don't think Tony Blair should have put any young kids out there."
Former foreign secretary Sir Malcolm Rifkind, a Conservative who initially opposed the war, said he was "deeply saddened" by the death but added Britain could not just leave Iraq.
I don't think Tony Blair should have put any young kids out there
Diane Douglas
Anguish of soldier's family
George Galloway MP, who wants all troops to be withdrawn from Iraq, said the latest casualty showed the "full disastrous consequences" of the war.
Former Labour MP Tam Dalyell, who is an honorary member of the Royal Scots Dragoon Guards officers' mess, also expressed his sadness.
"I am dreadfully sorry that this should have happened in a conflict where the military objective is far from clear," he said.
Some 230 British troops have been injured by enemy action since the conflict in the country began in 2003.
Of the 100 servicemen and women who have died in Iraq, 77 were classed as being killed in action.
The other 23 died from illness, non-combat injuries, accident or an unknown cause.
In all, 2,242 US troops have died in the conflict. There is no widely accepted figure for the number of Iraqi civilian deaths. Estimates have varied between 10,000 and 100,000.
The total death toll of the British Army in Iraq (excluding British soldiers employed privately as mercenaries) reached 100 this morning with a solider dying from injuries suffered in an explosion at Um Qasr. Um Qasr is an Iraqi port in the southern Basra province where British troops operate.

The Ministry of Defence has yet to name the soldier, a member of the 7th Armoured Brigade who are better known as "the Desert Rats".  They stated that the blast, which also injured three other soldiers, took place at around 8.30 a.m.

Sympathy has been widely expressed by various political and social figures. An official spokesman for British Prime Minister Tony Blair, who ordered the country's troops into Iraq, said that he was "deeply saddened" by the death, as he is by all fatalities of service personnel. Dr Liam Fox, the opposition Conservatives defence spokesman, also expressed his sympathies.

Critics of the war also expressed their sympathies but blamed the government for the soldiers' death. Reg Keys, father of a military policemen who died in an ambush in Iraq, said that "We have had 100 chances to learn our lesson. It just goes on and on." George Galloway, the Respect MP for Bethnal Green and Bow said "Our hearts go out to the family of the hundredth British soldier to die in Iraq. These one hundred deaths are the all too predictable consequences of the illegal and immoral war in Iraq. I don't want another British soldier to die occupying other people's countries." Respect echoed the call by the Stop the War Coalition for a demonstration in Parliament Square were the names of the all dead will be read aloud.

However, Conservative foreign secretary Sir Malcolm Rifkind, who originally opposed the war, claimed that Britain could not just leave Iraq.

Among countries that sent soldiers to Iraq, the UK ranks second in number of casualties, above Italy with 27 and Ukraine with 18, but is well below the US's figure of 2242. The number of Iraqi military casualties is agreed to be larger than this, but may lie anywhere between 5000 and 45,000 according to different sources.

The number of Iraqi civilian casualties from March 2003 to September 2004 is estimated as 100,000 (excluding the Fallujah attack), with most of the violent deaths due to air strikes.