Britain is facing an examination by the International Criminal Court over allegations that UK forces in Iraq were responsible for the "systematic abuse" of detainees.
The court's prosecutor Fatou Bensouda said she was staging a "preliminary examination" - the first step towards a formal inquiry - after receiving a dossier from a British law firm representing a number of detainees.
The Government indicated that it would co-operate with the inquiry by the court in The Hague - which was set up to investigate war crimes and crimes against humanity - while insisting it "completely rejects" the allegations of systematic abuse by British troops.
Officials expressed confidence that the prosecutor would not move to the next stage of launching a full investigation as the UK authorities were conducting their own inquiries into the allegations.
However they acknowledged that the case could hang over the country for years to come as the prosecutor's office satisfied themselves that the matter was being properly dealt with.
A statement on the court's website said that Mrs Bensouda had decided to re-open a preliminary examination previously concluded in 2006 after the law firm, Public Interest Lawyers, and the European Centre for Constitutional and Human Rights - a German human rights organisation - submitted fresh information earlier this year.
"The new information received by the office alleges the responsibility of officials of the United Kingdom for war crimes involving systematic detainee abuse in Iraq from 2003 until 2008," the statement said.
"The re-opened preliminary examination will analyse, in particular, alleged crimes attributed to the armed forces of the United Kingdom deployed in Iraq between 2003 and 2008."
Attorney General Dominic Grieve said that he understood the importance of the prosecutor following "proper legal procedures" when complaints were made he would her officials with "whatever is necessary to demonstrate that British justice is following its proper course."
"The Government completely rejects the allegation that there was systematic abuse carried out by the British Armed Forces in Iraq. British troops are some of the best in the world and we expect them to operate to the highest standards, in line with both domestic and international law," he said.
"In my experience, the vast majority of our armed forces meet those expectations. Where allegations have been made that individuals may have broken those laws, they are being comprehensively investigated."
The Director of Service Prosecutions Andrew Cayley QC - who is responsible for prosecutions of service personnel - said was confident that the case would not move to a formal investigation as the UK was already conducting its own inquiry through the Iraq Historical Allegations Team (IHAT) established in 2010.
Under the Statute of Rome which established the ICC, the court can only intervene in cases where there is no effective investigation by the national authorities.
"If the ICC is satisfied that the United Kingdom is genuinely investigating these crimes, they will allow us to do that. They may go on monitoring us for a number years in respect of investigations and prosecutions but they will not intervene," he said.
"Of course in the end it is for the prosecutor of the ICC to determine this, but I am confident based on the work that I've seen that IHAT has been doing, that the court will find that these are genuine criminal investigations that are taking place and they won't take it any further."
However Phil Shiner of Public Interest Lawyers dismissed the IHAT inquiries, saying that it was incapable of holding to account those at the top.
"We want a sea change. We don't want the UK to behave in this way. We think it reflects very badly on our armed forces and we think it reflects very badly on the nation," he told the BBC Radio 4 PM programme.
"We are very concerned that people at the very highest level knew exactly what was going on in Iraq and chose to turn a blind eye or - worse - actually sanctioned it."
Former defence secretary Geoff Hoon and former armed forces minister Adam Ingram are among those named in the file submitted by the firm.
The prosecutor's office will now study the dossier alongside submissions from the British Government and other relevant bodies - a process that could take several years before a final decision is taken on whether to move to a full investigation.
"The preliminary examination is the first step in a process," Mr Cayley said. "The preliminary examinations can carry on for a very lengthy period. there have been preliminary examinations going on for years."
The IHAT, led by a former civilian police detective, is currently looking into 53 cases of alleged unlawful killing and a further 93 cases of alleged mistreatment.
Public Interest Lawyers said they had compiled thousands of allegations of abuse amounting to "war crimes of torture" or to "cruel, inhuman or degrading treatment".
They were said to include beatings, burning, electric shocks and mock executions as well as sexual assault and humiliation, including forced nakedness and forced or simulated sexual acts, with "clear patterns" emerging of the same techniques being used again and again.
"There are considerable reasons to allege that those who bear the greatest responsibility for the crimes are situated at the highest levels, including all the way up the chain of command of the UK Army, and implicating former secretaries of state for defence and ministers for the armed forces personnel," the firm said in a statement.
Mr Grieve however said that he had seen no evidence of culpability by ministers or senior military officers with regard to the cases that were being investigated, and denied that involvement of the ICC was an embarrassment to the UK.
"If the ICC wishes to look at this and particularly to come and see what we are doing, I have no reason to be troubled by that at all. I don't even see it as embarrassing," he told the PM programme.