UK troops have killed Iraqi civilians including an eight-year-old girl when they were under no apparent threat, Amnesty International has claimed.
The Amnesty report will add to speculation about troops
The human rights group claims in "many" cases the deaths of civilians caused by UK troops had not been investigated.
Inquiries were "secretive" and handled by Military Police, its report says.
The claims are the latest in a string of allegations about the behaviour of coalition forces but ministers say many of them are already being investigated.
Separately on Tuesday the High Court ruled that relatives of 12 Iraqis allegedly killed by UK troops would get a full court hearing into the decision not to hold independent inquiries.
The families want the deaths declared a breach of the European Convention on Human Rights, but the UK Government says the convention does not apply in Iraq.
The concerns come after criticisms from the International Committee of the Red Cross emerged on Monday about the way coalition forces have treated prisoners.
BBC political editor Andrew Marr said some ministers were calling the claims against coalition forces a public relations "catastrophe", with particular worries about allegations against US troops.
Liberal Democrat foreign affairs spokesman Sir Menzies Campbell added that the UK was "paying a considerable moral price" for the reports as it would be answerable in the minds of Iraqis for anything done by anyone in the coalition's name.
The prime minister's official spokesman refused to say whether Tony Blair knew about widespread allegations of abuse by American forces before they were published in the media.
He added: "If the prime minister knew of anything which was going wrong, he would of course have acted accordingly."
Amnesty wants an independent civilian-led investigation into all alleged killings in Iraq.
But Defence Secretary Geoff Hoon told BBC Radio 4's World At One many of the cases had been raised already and were being investigated.
"If there are matters which require further investigations, those investigations will be pursued," he said.
"Moreover, if it is the case as a result of those investigations they may need to be further steps, such as the reference to an independent prosecutor, that again will happen."
Amnesty's UK director Kate Allen said there were 37 cases of Iraqi civilians being killed, but only 18 had been investigated.
She told BBC Radio 4's Today programme: "Killings by UK forces, in situations where they should not be using lethal force, are examined in secrecy and behind closed doors.
"Instead of the Army deciding whether to investigate itself when civilians are killed, there must be a full, impartial and civilian-led investigation into all allegations of killings by UK troops."
The report highlights the case of eight-year-old Hanan Saleh Matrud who was apparently shot by a member of B Company of the 1st Battalion of the King's Regiment last August.
'Shooting in the air'
An eye-witness told Amnesty the girl had been killed when a soldier aimed and fired from a distance of about 60 metres, and not when accidentally hit by a warning shot as claimed by the Army.
Colonel John Hughes-Wilson, a former British intelligence officer who served in the Gulf War, branded the Amnesty report "unfair".
"We are not talking about Tunbridge Wells here, we're talking about Dodge City, with no police force in the aftermath of a major war, with about 16 different armed groups going round shooting Iraqis," he told BBC Radio Five Live.
"If you're a soldier on the ground there, it must be absolutely terrifying."
He said the eight-year-old girl was hit by a stray shot in a riot and UK soldiers were not running amok.
The report also suggests families of those killed were often offered no information on how to lodge compensation claims.
But it does welcome attempts by the British authorities to strengthen the Iraqi police force.
Mr Hoon told MPs on Monday action was pending against British soldiers in two cases of alleged abuse.
He said he had only "very recently" seen a Red Cross report produced in February detailing further allegations.
The Ministry of Defence has denied Conservative suggestions that Armed Forces Minister Adam Ingram misled MPs when he said last week the government had received no report on human rights concerns.
It says he was referring to detainees, not treatment of civilians.
The government has also cast doubt on the authenticity of Daily Mirror pictures allegedly showing the assault of an Iraqi prisoner.
Ex-Downing Street media chief Alastair Campbell said editor Piers Morgan's position would be "untenable" if the photos were found to be fakes or had been staged.