The family of a Royal Military Police officer killed by a mob in Iraq is suing the Ministry of Defence for negligence.
Cpl Russell Aston, 30, was one of six Red Caps killed when they were set upon by a 400-strong mob in Iraq in 2003.
The claim alleges that commanders failed to take reasonable measures to keep their officers safe.
The Supreme Court ruled last month that families of soldiers killed in Iraq could seek damages from the government.
The judges ruled that the families could make damages claims under human rights legislation and sue for negligence.
The legal action launched by the family of Cpl Aston, who was from Swadlincote in Derbyshire, claims the MoD was negligent because the military police officers - known in the army as Red Caps - were not supplied with sufficient ammunition to defend themselves, or roadworthy vehicles or effective communications.
'Risk to life'
The Aston family's lawyer said it was expected that the families of the other men killed would join the legal action.
They were Sgt Simon Hamilton-Jewell, 41, from Chessington in Surrey; Cpl Paul Long, 24, of South Shields in Tyne and Wear; L/Cpl Benjamin McGowan Hyde, 23, from Northallerton in North Yorkshire; L/Cpl Tom Keys, 20, from Bala in North Wales; and Cpl Simon Miller, 21, from Washington in Tyne and Wear.
Solicitor-Advocate Simon McKay, who is acting for the families of Cpl Aston, Cpl Keys and Cpl Miller, said: "The claims follow a recent Supreme Court ruling extending the reach of the European Convention on Human Rights (ECHR) to some theatres of war and the scope of combat immunity as a defence available to the Ministry of Defence."
He said the claim alleged that the government breached Article 2 of the convention by failing to take the measures expected in light of the "real and immediate risk to life" of the soldiers.
He said it would be open to the MoD to argue the claims were out of time, but he hoped it would not try to "avoid accountability of a technical basis".
Cpl Miller's father John said: "We will be following suit and have been speaking to the lawyer.
"We want the MoD to accept accountability, which will give us a sense of justice. If the MoD thinks we are going to go away, they are sorely mistaken."
Last month, Defence Secretary Philip Hammond said that he was concerned at the "wider implications" of the Supreme Court's judgement.
He said the ruling could "ultimately make it more difficult for our troops to carry out operations, and potentially throws open a wide range of military decisions to the uncertainty of litigation".
Mr Hammond added: "We will continue to make this point in future legal proceedings as it can't be right that troops on operations have to put the European Convention on Human Rights ahead of what is operationally vital to protect our national security."
A spokesman for the MoD said on Wednesday: "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."