Law firm Public Interest Lawyers (PIL) have 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.
In a statement PIL said:
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.
A statement on the International Criminal Court website said that prosecutor Fatou Bensouda, had decided to re-open a preliminary examination into abuse in Iraq by UK troops. It previously concluded in 2006, following submission of further information to her office 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 has said the Government "completely rejects" the claim that UK forces have been responsible for systemic abuse and would do "whatever is necessary" to show any allegations were being dealt with within the British justice system.
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.
As the minister responsible for overseeing the UK's prosecutors, I understand the importance of the ICC [International Criminal Court] prosecutor following the proper legal procedures when complaints are made.
– Attorney General Dominic Grieve
He said that the Government "has been, and remains, a strong supporter" of the ICC. He added that he would provide the office of the prosecutor with "whatever is necessary to demonstrate that British justice is following its proper course".