The ICC will examine separate allegations, mostly from former detainees held in British miltiary custody in Iraq. Photograph: Ian Waldie/Getty Images
Allegations that British troops were responsible for a series of war crimesafter the invasion of Iraq are to be examined by the international criminal court (ICC) at The Hague, the specialist tribunal has announced.
The court is to conduct a preliminary examination of what have been estimated to be 60 alleged cases of unlawful killing and claims that more than 170 Iraqis were mistreated while in British military custody during the conflict.
British defence officials are confident that the ICC will not move to the next stage and announce a formal investigation, largely because the UK has the capacity to investigate the allegations itself.
However, the announcement is a blow to the prestige of the armed forces as the UK is the only western state that has faced a preliminary investigation at the ICC. The court's decision places the UK in the company of countries such as the Central African Republic, Colombia and Afghanistan.
In a statement released on Tuesday, the ICC 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."
But Dominic Grieve, the attorney general, said the government rejected any 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."
Grieve added that, although the allegations were already being "comprehensively investigated" in Britain, "the UK government has been, and remains, a strong supporter of the ICC and I will provide the office of the prosecutor with whatever is necessary to demonstrate that British justice is following its proper course".
The investigation means there will be a degree of scrutiny from The Hague of the British police team responsible for investigating the allegations, as well as the Service Prosecuting Authority (SPA), which is responsible for bringing courts martial cases, and Grieve, who must make the final decision on war crimes prosecutions in the UK.
The process of a preliminary examination can take several years.
The newly appointed head of the SPA, Andrew Cayley QC, who has 20 years' experience of prosecuting at war crimes tribunals in Cambodia and at The Hague, said he was confident that the ICC would eventually conclude that the UK should continue to investigate the allegations. Cayley said the SPA "will not flinch" from bringing prosecutions if the evidence justified it.
He added that he did not expect any civilians – officials or government ministers – would end up facing prosecution.
Any war crime committed by British servicemen or servicewomen is an offence under English law by virtue of the International Criminal Court Act 2001.
The ICC has already seen evidence suggesting that British troops did commit war crimes in Iraq, concluding after receiving a previous complaint in 2006: "There was a reasonable basis to believe that crimes within the jurisdiction of the court had been committed, namely wilful killing and inhuman treatment."
At that point, the court concluded that it should take no action, as there were fewer than 20 allegations.
Many more cases have emerged in recent years. Currently, the Iraq Historic Allegations Team, the body set up by the Ministry of Defence to investigate complaints arising from the five-year British military occupation of the south-east of the country, is examining 52 complaints of unlawful killing involving 63 deaths and 93 allegations of mistreatment involving 179 people.
The alleged unlawful killings include a number of deaths in custody and the complaints of mistreatment range from relatively minor abuse to torture.
PIL withdrew allegations of unlawful killings arising out of one incident, a firefight in May 2004 known as the battle of Danny Boy, although an inquiry continues to examine allegations that a number of insurgents taken prisoner at that time were mistreated.
The ICC will examine separate allegations, mostly from former detainees held in Iraq. Following the death of Baha Mousa, one soldier, Corporal Donald Payne, admitted being guilty of inhumane treatment of detainees and was jailed for one year. He became the first and only British soldier to admit a war crime.
Six other soldiers were acquitted. The judge found that Mousa and several other men had been subjected to a series of assaults over 36 hours, but a number of charges had been dropped because of "a more or less obvious closing of ranks".
The MoD admitted to the Guardian four years ago that at least seven further Iraqi civilians had died in UK military custody. Since then, no one has been charged or prosecuted.
• This article was amended on Tuesday 13 May 2014 to reflect the fact that the ICC is not an EU institution, and to remove a reference to the forthcoming European elections.