The family of one of the six Red Caps murdered by a mob in Iraq is suing the Ministry of Defence for negligence.
Corporal Simon Miller, of Washington, and Corporal Paul Long, of South Shields, were part of a team of six military policeman slaughtered at a police station by an Iraqi mob.
The case, the first of its kind, is being brought by the relatives of one of their team, Corporal Russell Aston.
The claim could cost the MoD more than £250,000.
It follows a landmark judgement by the Supreme Court last month which ruled that soldiers at war in foreign lands are covered by human rights laws and owed a duty of care.
The new claim on behalf of Cpl Aston's family alleges that commanders failed to take reasonable measures to keep the military policeman safe when he and five others went into a hostile town near Basra in June 2003 and were attacked by hundreds of violent locals who brutally murdered them all.
The details of the claim allege military chiefs were negligent because they failed to supply sufficient ammunition to enable the Red Caps to defend themselves when they arrived at the police station in Majar al Kabir and were confronted by the mob.
It also claims commanders failed to supply roadworthy vehicles and failed to supply effective communications to the Royal Military Police.
The five men who died alongside Cpl Aston were Lance Corporal Thomas Keys, Sergeant Simon Hamilton-Jewell, Corporal Simon Miller, of Washington, Lance Corporal Ben Hyde and Corporal Paul Long, of South Shields.
An inquest into their deaths in 2006 recorded a verdict of unlawful killing and heard evidence the soldiers had too little ammunition, old radios and no satellite phone when they were ambushed.
John Miller, whose son Simon was killed in the attack, said he too will sue the Ministry of Defence.
"Simon's patrol should have had a satellite phone and 150 rounds of ammunition; they were denied both, they couldn't call for help, they couldn't initiate a firefight because they didn't have enough ammunition against a mob of 500 firing RPGs and AK-47s.
"Can anyone imagine that situation and be totally useless under it? All we want is an admission of guilt from the MoD. I'm so angry and very, very hurt.
"It could be classed as a blunt sword, but all I've wanted is to get these people into court and for the MoD to accept the failings that were identified by the board of enquiry.
"Nothing else would give me greater satisfaction, and I know nothing will bring my amazing son back, but if we could get that justice and admission of guilt, my wife and I could turn the chapter and pick up the threads of the life we once had."
Despite arresting eight Iraqis in connection with the killings in 2010, no one has been convicted of the murders and repeated requests for a public inquiry have been turned down.
The new claim is seeking damages on behalf of Cpl Aston's family - he had a young daughter when he was killed.
It is understood to have been lodged on Tuesday night.
If the Aston family claim is successful it could pave the way for many more similar actions from other soldiers, or their families, who have been killed or injured in battle and feel more should have been done to keep them safe.
An MoD spokesman said: "Our thoughts remain with the families of those who lost their lives in this incident. However, it would be inappropriate to comment on any forthcoming legal proceedings."