John Miller, father of Cpl Simon Miller, has also been involved in this new development, which he says has taken months to plan.

He said: “This is a massive step forward. It’s come about because last July the European Court of Human Rights placed the police station where Simon and the others were killed was put under the jurisdiction of the UK.

“It was placed there because of the investigation into the abuse of Iraqi prisoners, but now it can work the other way because it puts the lives of the Red Caps in UK jurisdiction.

“We say that the UK failed to protect them and that there hasn’t been an adequate independent investigation, and we want one to take place.”

Mrs Long continued: “The start of this long letter cites four parts of the European Convention on Human Rights. What we say is that the UK failed in its duty to protect my son’s life and his colleagues’ lives and that it needs investigation.

“It’s my name and Paul’s throughout the letter but I am doing this for all the families.

“And I fully recognise that this is just the start and it could take years to get justice for Paul ... but that’s all I have ever wanted.”

An MoD spokesman 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.”