The Royal Military police have launched a criminal investigation into allegations that British soldiers repeatedly raped and mutilated an Iraqi civilian, the Guardian has learned.

Next week, lawyers for the man will issue legal proceedings against the government in the British civil courts. Their barrister is Sir Ken Macdonald QC, who was the top prosecutor in England and Wales until last year. The government has ordered an investigation into the claims, but is refusing to hold an independent inquiry.

The man was working for the British army as a labourer at Camp Breadbasket in Basra, the scene of other abuse allegations. In May 2003, he was caught up in a raid to catch people stealing supplies.

He alleges that two soldiers raped him, subjecting him to a 15-minute ordeal, then slashed his arms and body with a knife. He was treated in hospital for the cuts and the Royal Military police are understood to have secured the medical records. The victim was 18 at the time and said he was so traumatised he tried to kill himself.

The man does not want his real name to be known but is called Samir in his witness statement. He claims he was attacked after accidentally entering a room where two male soldiers were engaged in a sexual act. His witness statement says: “The room was fairly small, perhaps about four metres by four metres.”

He says one soldier pulled him into the room and punched him in the face, while another blocked the exit: “The man who had punched me was stronger than me, he grabbed me by the neck and kicked my legs from under me and I fell to the floor. I fell on my back. He took out a knife and held it against my neck … I was terrified that they were going to kill me at this point. The other soldier started to rip my trousers down … I started screaming. He then pulled my underwear down. All this time the other man was holding me down and putting the knife against my throat. They then flipped me on to my stomach.”

Samir says both soldiers then took it in turn to rape him, then slashed him with the knife, leaving him in “unbearable” agony. He says he left the room and other soldiers helped him to get medical attention for the knife wounds.

His solicitor, Daniel Carey, says witnesses confirm the victim was taken into a room by British soldiers, only to return looking distressed.

Carey also said: “The psychiatrist’s, conclusions were that he was suffering from depression, post traumatic stress disorder and anxiety as a result of the incident. We also interviewed two other Iraqis who were present with [the victim] at the base and saw him enter the room where he says he was raped and witnessed his distress and injuries following the attack.”In the witness statement, Samir says: “It fills me with such shame to talk about this, even with my solicitors. I have broken down telling them of what happened.”

Samir said he had not told his wife or family what allegedly happened to him.

Mazin Younis interviewed the man for the Iraqi League, a human rights group, and said: “Because the shame of rape in the conservative culture of Iraq is so pervasive, most Iraqis you talk to tell you they would much prefer death. It took immense bravery on the part of Samir to come forward and talk.

“Samir was not an insurgent. He was never a member of the Ba’ath party. He was simply accused of being a thief in a war-torn country, and now he must live with the trauma of rape for the rest of his life.”

The British government, in letters seen by the Guardian, is trying to have the legal case struck out. It says the fact the victim waited six years to make his claim means he has not brought his case in time.

Carey said: “I deplore the government’s cynicism in seeking to avoidany accountability by relying on technical rules of the court about the delay in bringing the claim.”

The armed forces minister, Bill Rammell, said: “Allegations must not be taken as fact and formal investigations must be allowed to take their course without judgments being made prematurely.”

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