One portrays an Iraqi mother and young child, kneeling on the floor, as if in prayer. They have been shot dead at close range.

The pictures show other victims, shot execution-style in the head and chest in their homes. An American government official said they revealed that the marines involved had “suffered a total breakdown in morality and leadership”.

The killings are emerging as the worst known American atrocity of the Iraq war. At least seven women and three children were among those killed. Witness accounts obtained by The Sunday Times suggest the toll of children may be as high as six. “This one is ugly,” a US military official said.

In Britain, the chief of the defence staff, Air Chief Marshal Sir Jock Stirrup, said yesterday that the “appalling” reports of the massacre could undermine British support for the war. “This sort of accusation does make that harder to achieve,” he said.

The pictures of the dead, which are being closely guarded by the US naval criminal investigation service, were taken by a military photographer who is believed to have arrived on the scene moments after the shootings.

Many American forces are accompanied by photographers to gather intelligence and to shield soldiers from false accusations of torture, intimidation and violence. In this case, the evidence points fatefully to a murder spree by marines.

The stain on the American military could prove harder to erase than the photographs of sadistic prisoner abuse at Abu Ghraib.

Comparisons are being made to the My Lai massacre in 1968 in Vietnam, in which American soldiers slaughtered up to 500 villagers.

Up to a dozen marines may face criminal charges including murder, which carries the death penalty, dereliction of duty and filing a false report. Three marine commanders were suspended last month.

The naval inquiry is focusing on the actions of a sergeant who may have been the leader of a four-man “fire team”.

Miguel Terrazas, 20, a lance-corporal from El Paso, Texas, was travelling in a convoy of four Humvees in Haditha just after 7am on November 19 last year when a roadside bomb struck his vehicle, killing him and wounding two others.

The events that followed are the subject of two military inquiries due to report soon: one into the facts, the other into a cover-up.

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