Tony Blair meets a member of the RAF regiment after the memorial service. He was later snubbed by the father of a dead soldier. Photograph: Jonathan Brady/EPA

Six-and-a-half years after he sent British forces into battle in Iraq, Tony Blair today came face to face with the uncomfortable consequences of his decision when the father of one of the 178 military personnel who died in the conflict refused to shake his hand, denouncing him as a "war criminal".

Peter Brierley, whose son Lance Corporal Shaun Brierley died in a road accident while on service in Iraq barely a week after the invasion in March 2003, publicly snubbed the former prime minister at a reception that followed a service commemorating those who served in the conflict. As Blair proffered his hand, Brierley told him: "I'm not shaking your hand, you've got blood on it."

Later, Brierley, from Batley, West Yorkshire, who has campaigned for a number of years for an inquiry into the war, said: "I believe Tony Blair is a war criminal. I can't bear to be in the same room as him ... I believe he's got the blood of my son and all of the other men and women who died out there on his hands."

The incident topped off a discomfiting day for Blair, who had previously heard Rowan Williams, the Archbishop of Canterbury, speak pointedly at the service at St Paul's cathedral, London, of the "mistakes" of those behind the conflict, including those "policymakers" who had lost sight of the cost of war.

"Many people of my generation and younger grew up doubting we should ever see another straightforward international conflict, fought by a standing army with conventional weapons," Williams said. "We had begun to forget the realities of cost. And when such conflict appeared on the horizon, there were those among both policymakers and commentators who were able to talk about it without really measuring the price, the cost of justice."

The prime minister, Gordon Brown, and Iraq's president, Jalal Talibani, were joined at the service by a dozen senior members of the royal family, including the Queen, the Prince of Wales and Prince William, bereaved family members, and 2,000 of the 100,000 personnel who served in Iraq during Britain's six-year engagement, which formally ended in April this year.

But it was the suntanned man sitting in the row behind his Downing Street successor whose presence excited most comment, and who perhaps felt the archbishop's words most keenly.

Williams criticised the "invisible enemies – letting ends justify means, letting others rather than oneself carry the cost, denying the difficulties or the failures so as to present a good public face" – that had menaced those involved in the conflict.

Williams is blessed with a sonorous lilt and a subtle prose style that can sometimes make his words, at first, seem more emollient than they are. This was no soothing homily from a man who has previously described decisions leading to Britain's involvement in the war as being morally and practically flawed.

"Reflecting on the Iraq campaign, we cannot say that no mistakes were ever made – when has that ever been the case?" Williams said. "But we can be grateful for the courage and honesty shown in facing them. Justice does not come without cost. In the most obvious sense, it is the cost of life and safety. But there is another sort of cost involved in holding back the easy instinctive response and checking that you are genuinely doing something for the sake of long-term building or healing."

He alluded to the vexed questions those responsible had faced, however, even as he emphasised that the justice of the war itself remained under debate.

"In a world as complicated as ours has become, it would be a very rash person who would feel able to say without hesitation, this was absolutely the right or the wrong thing to do, the right or the wrong place to be," he said.

Only once, and then obliquely, were the uncounted mass of Iraqi dead referred to, when the archbishop prayed for "those whose memory we cherish, and those whose names we will never know".

Monarchs, princes, politicians and soldiers are dwarfed equally under the dome of St Paul's. It was collectively, in its clumps of khaki, navy and air force blue, that the congregation had its force. In a sea of sober suits and sharply ironed uniforms, what stood out were the scarlet sashes, the gold epaulettes, the thousands of polished medals.