Families of the 51 Britons killed were in attendance

They were joined by senior members of the Royal Family, including the Queen and Prince Charles.

Government ministers including Tony Blair, leaders of the armed forces and leaders of world faith organisations were also present.

In his address, the Archbishop of Canterbury, Dr Rowan Williams, urged coalition leaders to check the "integrity" of their mission in Iraq was not under strain.

But the emphasis of Friday's service, attended by 2,000 people, was on remembrance

The service began with prayers for all those who were killed, British and Iraqi alike.

Then Dr Williams addressed the congregation, telling the 250 relatives present: "You know the cost [of war] in a unique way."

Let him (Tony Blair) see the misery that his decision to go to war has caused Reg Keys Father of dead serviceman Service in pictures

Some of the families had said they feared their loved ones may have died in vain, and had called on the prime minister to stay away from the service.

Dr Williams said: "We have to go back and test what has happened in the light of the original vision; we have to find out what we have learned, what now looks different, where our integrity has been stretched or challenged."

He reminded UK and US leaders they would be held accountable for what was happening in Iraq.

But he also condemned those who ignored world oppression without intervention.

The Bishop to the Forces, the Rt Rev David Conner, led prayers for peace and for "all who work in great danger day by day" in Iraq.

Tears

The Duke of Edinburgh and Air Chief Marshal Brian Burridge read the lessons before a specially composed anthem sung by the cathedral choir.

We have to find out what we have learned Archbishop of Canterbury Dr Rowan Williams' address in full

The Bishop of London, the Right Rev Richard Chartres, led prayers for penitence and reconciliation.

Roman Catholic Archbishop of Westminster Cardinal Cormac Murphy-O'Connor and Methodist Conference vice-president Judy Jarvis called for "an end to injustice, terrorism and war".

The traditional tune Last Post preceded two minutes' silence, broken by buglers sounding Reveille.

Before the service, some families voiced their disquiet at the government, including Defence Secretary Geoff Hoon, being so involved in the service.

BRITISH MILITARY DEATHS IN IRAQ 51 British troops have died 21 were killed in combat 19 died in accidents Five troops were killed by 'friendly fire' Four died of natural causes Two deaths are under investigation British casualties in Iraq How should we remember our dead?

Reg Keys, of north Wales, whose son Tom died alongside five other military policeman six weeks after the end of hostilities, was bitter that Mr Blair was attending the service.

"Let him see the misery that his decision to go to war has caused. Give the man a front seat but I won't sit next to him," he said.

However Peter Brierley said his son Shaun, who was killed in March, justified the war because Saddam Hussein had been removed.

After the service, he said it had been a "wonderful" experience.

"Hopefully, there will have been enough prayers said to get the peace that we really should have around the world," he said.

The Queen as head of all the forces led the Royal Family

Royal Marines and Navy spokesman Ben Curry said it had been a "calm and peaceful service".

"I lost a couple of good friends in an early helicopter crash and it was a fitting service for them and for the nation," he said.

"It wasn't a thanksgiving service but it was important to say thank you to all the services involved.

"It was an act of remembrance and was very poignant. There were a lot of different emotions for different people."

According to Ministry of Defence figures, 51 soldiers have died in Iraq - 21 killed in action, 19 in accidents and five in cases of so-called friendly fire.

A further four died of natural causes, while two deaths are still under investigation.

Unlike after previous conflicts, no victory parade or military march past has been held.