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 inquiry will be the first time the UK has been the subject of an ICC investigation.
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.
However, they said that the case could hang over the country for years as the prosecutor’s office satisfied itself that the matter was being properly dealt with. A statement on the court’s website said that Mrs Bensouda had decided to reopen 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 statement said: “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 reopened 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”.
But he added: “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.
“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 he was confident that the case would not move to a formal investigation as the UK was already conducting its own inquiry through the Iraq Historic 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 national authorities.
Mr Cayley said: “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.”
However Phil Shiner of Public Interest Lawyers dismissed the IHAT inquiries, saying that it was incapable of holding to account those at the top.
He added: “We want a sea change. We don’t want the UK to behave in this way.”