Corporal Gordon Alexander Pritchard, 31, served with the Royal Scots Dragoon Guards. He died from injuries sustained in an explosion in Um Qasr in British-patrolled Basra province.

The blast at 8.34am local time (0534 GMT) also injured three other soldiers, one seriously, the Ministry of Defence said.

Corporal Pritchard was commanding the lead Land Rover in a three-vehicle rations and water run when the explosion went off.

Tony Blair, who ordered British troops to join the invasion, was "deeply saddened" by the death, as he was by all fatalities of service personnel, his official spokesman said.

The spokesman said British forces in Iraq - which currently amount to around 8,000 troops - would stay in the country for "as long as necessary".

"No life is worth this kind of sacrifice but, in terms of why we are in Iraq, we have now had three democratic elections in a country that was brutalised for decades," he told reporters.

Critics of the war blamed the government for the soldiers' deaths. Reg Keys, whose son Thomas was one of six military policemen killed in an ambush in Iraq more than two years ago, said the milestone figure was "absolutely dreadful".

"We have had 100 chances to learn our lesson. It just goes on and on," he said.

"These deaths were 100% preventable. These lads are dying for a falsehood. Their oath of allegiance has been betrayed. This was not what they went to war for. They are not the world's police."

George Galloway, the Respect MP for Bethnal Green and Bow and a leading critic of the war, said: "What a melancholy milestone this is.

"The United States long ago passed it - now 2,000 deaths and counting. Of course, nobody is counting the number of Iraqis killed.

"These 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. That's why all British troops must be brought home now."

The 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. Dr Liam Fox, the Tory defence spokesman, also expressed his sympathy over today's fatality.

Second British loss in 24 hours

Today's fatality is the second British loss in just over 24 hours. Lance Corporal Allan Douglas, 22, of the 1st Battalion the Highlanders, died after being wounded by sniper fire in Maysan province, south-east Iraq, yesterday.

His father, Walter, of Aberdeen, told the Daily Mirror 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 yesterday.

Of the 100 fatalities, the MoD classes 77 soldiers as having been killed in action and 23 as having died from illness, non-combat injuries, accidents or unknown causes. At least 230 British troops have been injured. British troops are regularly targeted by insurgents using improvised explosive devices, which are often left by roadsides to target patrols.

In all, just over 4,000 people, including Iraqis and British civilians as well as servicemen and women, have been evacuated to the UK for medical treatment. The vast majority suffered illness or an accident.

It is hoped the number of British troops in Iraq will eventually be scaled back as UK and US forces train Iraqi security personnel to take their place but no definite withdrawal plan has been announced.

Mr Reid last week announced plans for a major new deployment of British troops to a dangerous area in southern Afghanistan, where US forces have been targeted by suicide bombers.

In the next few months, around 3,300 combat troops will be sent to Afghanistan, on top of around 850 already there, and will take on a Nato peacekeeping and counter-narcotics mission.