John, not his real name, spent much of his career in the Special Investigation Branch, the Royal Military Police's equivalent of the CID.
But it was a career he felt he was forced to abandon after concluding that the RMP's standards were increasingly at odds with his own moral values.
"For too long I belonged to an organisation that wasn't seeking out the truth."
The turning point for him was a series of suspicious deaths of Iraqi citizens.
One case which came to his attention involved the alleged murder of an Iraqi by a British sergeant.
There was, he claims, evidence to strongly suggest that the Iraqi victim had been shot at point blank range for merely throwing rocks at a British army tank.
Baha Mousa and family. The hotelier died in custody of British forces.
"A friend of mine who was a senior NCO went to the scene and was ordered not to investigate it as a murder. He was told that statements should just be taken as if it was a routine incident."
Some cases involving allegations of murder and abuse, followed by a failure to carry out an adequate investigation, have already come to public attention.
The death of Baha Mousa, an Iraqi hotel receptionist who died in British military custody having allegedly been beaten by soldiers, is currently the subject of a public inquiry.
The terms of reference for another inquiry, into the alleged death in custody of Hamid Al-Sweady, will be announced on Friday.
Abuse is rare
The Ministry of Defence insists that any substantive allegations of abuse brought to its attention will always be investigated and that such cases are relatively rare.
"We must remember that over 100,000 of our personnel served in Iraq and, with the exception of a few individuals, they have performed to the highest standards under extraordinarily testing conditions there," said an MoD statement responding to the allegations.
Any substantive allegations of abuse brought to our attention will always be investigated and such cases are relatively rare
Ministry of Defence
John agrees that the vast majority of British soldiers have served their country with distinction.
However, he believes that serious cases of suspected abuse are far more common than the army is prepared to admit.
"I've seen documentary evidence that there were incidents, running into the hundreds, involving death and serious injury to Iraqis," he claims.
"It is the actions of a few who have been shown to be bad apples. But the system is so flawed and some of the decision making has been so perverse that it is fair to say that the barrel is probably rotten."
Out of depth
In recent decades, the bulk of military police work has involved dealing with the alleged crimes of soldiers stationed at bases in the UK and Germany.
"Soldiers fighting each other, the odd drink-drive, a bit of theft here and there," were the staple source of investigations according to John.
Whistleblower 'John' says the majority of troops serve with distinction
But when troops are deployed to theatres of war and peacekeeping roles abroad, the RMP goes with them.
In the case of Iraq, this meant the RMP suddenly found itself having to investigate alleged war crimes.
"We saw the military police - and the Special Investigation Branch in particular - being massively out of its depth," says John.
The most serious cases arising out of the British involvement in Iraq have, according to John, been characterised by RMP "blunders, mistakes, ineptitude and the course of investigations being bent to serve the real or perceived interests of the chain of command of the army" rather than the interests of justice.
Toe the line
A lack of resources, he claims, is partly to blame. But, he believes, there are also serious structural flaws in the army justice system.
Senior officers in the mainstream army have to be informed when the RMP mounts any investigation into serious allegations.
This, says John, gives the army hierarchy the opportunity to interfere or to make life difficult for the investigators by, for example, refusing to provide transport to the scene of an alleged crime.
"There is a common misconception that the Royal Military Police are in some way independent of the army.
In fact they are very, very closely wrapped into the chain of command and that is why so many things go wrong."
The dark picture John paints of a military police force infected by a culture of ineptitude and cover-up was dramatically reinforced last week when three high court judges accused the RMP's second in command, Colonel Dudley Giles, of being "a most unsatisfactory witness".
In a damning judgment concerning the RMP's failure to disclose documents relating to the alleged murder of Hamid Al-Sweady, the judges concluded that Colonel Giles "lacked the necessary objectivity, proficiency and reliability" expected of an official government witness.
The motto of the Royal Military Police is "By example, shall we lead". But with the trustworthiness of one of the force's most senior officers now in doubt, the RMP will be under more pressure than ever to set a higher standard.
You can hear the full report on the Donal MacIntyre programme on 5 live on Sunday, 11 October, 2009 at 1930 BST. Download the free podcast.You can can contact the programme by emailing firstname.lastname@example.org
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