Corporal Paul Long, 24, was one of six Red Caps murdered when about 400 Iraqis attacked a police station in Majar al-Kabir, Maysan, southern Iraq, in June 2003.
There have been a series of investigations into what went wrong, including an internal Army Board of Inquiry and an inquest.
But Cpl Long’s mother Pat, of Hebburn, South Tyneside, believes those reviews were inadequate and said she still has unanswered questions about what happened to her son.
Her legal team, from Birmingham-based Public Interest Lawyers, has now written to Defence Secretary Philip Hammond calling for a fresh inquiry and warning that they could bring a judicial review challenge in the High Court.
The 25-page letter states: “At present, Mrs Long feels a strong sense of injustice, that the questions she has about her son’s death have not been properly answered and that, importantly, the lack of any accountability for the situation in which her son was placed does not do justice for her son.”
It continues: “The circumstances of the deaths in this case reveal obvious defects in the systems which ought reasonably be expected to be in place to protect the lives of British soldiers.”
The letter calls for an independent inquiry with terms of reference broad enough to cover operational planning and the provision of equipment to British soldiers in Iraq.
It says it is “extremely doubtful” whether a fresh inquest would be the appropriate way of meeting the Government’s obligations under Article 2 of the European Convention on Human Rights, which protects the right to life.
The letter notes: “Mrs Long does not accept that there has been adequate investigation into the circumstances of her son’s death.
“She takes the view, therefore, that a full and proper understanding of her son’s death has not been realised.”
The murders of the six Red Caps - Cpl Long, from South Shields, Tyne and Wear; Sergeant Simon Hamilton-Jewell, 41, from Chessington, Surrey; Corporal Russell Aston, 30, from Swadlincote, Derbyshire; Corporal Simon Miller, 21, from Washington, Tyne and Wear; Lance Corporal Benjamin Hyde, 23, from Northallerton, North Yorkshire; and Lance Corporal Tom Keys, 20, from Bala, North Wales - shocked UK forces in Iraq.
The British military policemen had been training local Iraqi officers when the police station came under attack on June 24, 2003.
An inquest in March 2006 heard that some of their bodies were found riddled with bullets, while others had marks that suggested they had been dragged, tied up or beaten with rifles.
The hearing was told that the Red Caps were not equipped with Iridium satellite telephones to call for help, despite a direct order from their commanding officer that all units leaving base must carry one.
This was due to supply shortages because there were just 27 of the telephones available to the 1,000-strong 1 Para battle group.
Another direct order that each man should carry 150 rounds of ammunition had also been ignored and the six Red Caps were carrying only 50 rounds each.
Oxfordshire Coroner Nicholas Gardiner recorded a narrative verdict of unlawful killing, saying the six soldiers should have been better equipped but their deaths could not have been avoided.
Their families’ long battle for justice suffered a major setback when charges against two Iraqi men accused of the murders were dropped in October 2010.
A MoD spokeswoman said: “We can confirm that we have received a pre-action protocol letter from Public Interest Lawyers regarding the deaths of six Royal Military Police in Iraq in 2003.
“We will consider the letter carefully and respond to Public Interest Lawyers in due course. Our thoughts remain with the families of the six soldiers who lost their lives.”