The team investigating allegations UK troops abused Iraqi civilians has been called "a shambles".
Lawyers for the alleged victims say only one has been interviewed so far, despite the Iraq Historic Allegations (IHAT) team starting work in November.
Complainants are now refusing to co-operate - they say the investigators will not follow guidelines for interviewing vulnerable witnesses.
IHAT has denied these claims and says there is no basis for their decision.
The IHAT team has a staff of 83, of which 38 have been provided by private company G4S.
It will cost £7.5m over two years and aims to take statements from more than 140 alleged victims.
The Iraqi civilians complain the abuse happened between 2003 and late 2008. The allegations include sexual abuse, deprivation of food, water and sleep, prolonged solitary confinement and mock executions.
Phil Shiner from Public Interest Lawyers, who is representing all of them, said: "It's been a complete and utter shambles, it must have cost the taxpayer millions."
He says IHAT has only managed to interview one of his clients, which he said is "grossly incompetent".
Mr Shiner says the first attempt to take statements from Iraqis in Lebanon ended when investigators were asked to leave by the authorities there.
A second attempt in Turkey ended when an interviewee objected to the questioning. He cut short the interview and flew home to Iraq.
"It was a multiple of serious errors they made. They clearly haven't been trained," he said.
He said the law classifies his clients as "vulnerable and intimidated witnesses".
Despite an agreement with IHAT to use civilian Achieving Best Evidence guidelines, he said investigators failed to follow them.
"I have a duty to them. I could not advise them, some of whom have been raped, that they should go through this," he said.
They pulled out of the process, which means no more interviews can take place.
Mr Shiner has called for a public inquiry, but denies he has tried to sabotage IHAT.
The head of the team, retired Det Ch Supt Geoff White, said he would not enter a "public debate" on the issues because he did not want to prejudice any future proceedings.
However, in a statement he said: "Our aim is to get to the bottom of what is alleged to have occurred with a view to deciding if there is sufficient evidence to refer cases for potential prosecution.
"I simply do not accept that the difficulties we now face stem from failings on the part of IHAT.
"We have taken advice from an experienced Queen's Counsel and his conclusion is that there was no reasonable basis for Public Interest Lawyers decision to advise complainants to withdraw from the interviews."
For now, no more interviews can take place.
Two major inquiries into allegations of abuse by British forces in Iraq are already under way and there have also been a series of prosecutions.
In 2007 Cpl Donald Payne became Britain's first UK soldier to be convicted as a war criminal after pleading guilty to inhumanely treating civilian detainees in Basra in 2003.
Payne was jailed for a year and dismissed from the army.
He was accused of charges relating to the abuse of the group of Iraqi men arrested at a hotel where weapons and suspected bomb-making equipment were found.
The detainees included Iraqi receptionist Baha Mousa, 26, who later died while in British custody - the report from the inquiry into his death is due to be published in September.
But there are those in the military who doubt that IHAT can offer anything new.
"The people making these accusations have to put up or shut up," said Adrian Weale, of staff association the British Armed Forces Federation.
"It's almost impossible to imagine that justice can be done after this amount of time has passed," he added.
His conclusion is that it is a "waste of time".
So far IHAT has cost £1.4m.
Next month it will come under even closer scrutiny at the Court of Appeal.
Phil Shiner is set to argue that the only way to find out what really happened to his clients is to hold a full public inquiry.