The High Court has heard that following the Chilcot inquiry, which concluded that the invasion of Iraq was unnecessary and undermined the UN, Tony Blair should be prosecuted.

Michael Mansfield QC argued in his opening remarks that the offence of waging an aggressive war has effectively been assimilated into English law.

But it’s not just Tony Blair who is being called to account. Former foreign secretary Jack Straw and former attorney general Lord Goldsmith are also being earmarked for a court appearance by former Iraqi general Abdulwaheed al-Rabbat. According to Mansfield, he was motivated by the publication of the Chilcot report last year.

Mansfield summarised the report’s findings as: “Saddam Hussein did not pose an urgent threat to the UK, intelligence reporting about [Iraqi] weapons of mass destruction was presented with unwarranted certainty, that the war was unnecessary and that the UK undermined the authority of the UN security council.

“Nothing could be more emphatic than these findings, it was an unlawful war.” he said.

He went on to draw comparisons with the Nuremberg trials against the Nazi leadership. He argued that when the British prosecutor Sir Hartley Shawcross opened the cases, he acted as though the crime of aggression was already a part of English law.

The first application to prosecute Blair was dismissed at Westminster magistrates court on the grounds that he enjoys immunity and that the crime of aggression does not exist in English law. It also heard that attempts to get the international criminal court to prosecute have not made any significant progress.

The high court also heard that the ICC finally defined the crime of aggression in 2010, however, a sufficient number of states have not yet ratified it. The appeal to the high court by Rabbat is being heard by the lord chief justice, Lord Thomas of Cwmgiedd, and Mr Justice Ouseley.

James Eadie QC, speaking on behalf of the current attorney general, Jeremy Wright QC, who has intervened in the case, argues that the crime does not exist in the statute book. “The crime of aggression is not known to English law,” he told the court.

Eadie also cited the case of a 2006 House of Lords ruling of R v Jones which found unanimously that the international law crime of aggression is not an offence under UK domestic legislation.

A number of Rabbat’s supporters were at the back of the court. Just before the hearing, Sabah al-Mukhtar, of the Arab Lawyers Network said: “This is just looking at whether the first court was right in refusing to entertain the case.

“The magistrates’ court dismissed it on the grounds that Tony Blair had immunity and that the crime of aggression was not part of English law. Many think they were not correct on that.

A former chief of the Iraqi army, Rabbat himself was not in court. He lives in Muscat, the Omani capital where it is rumoured he doesn’t own a passport.

His lawyers have stated that he has been able to bring the case to British courts because Iraq was previously under British control during the occupation. He is therefore considered to have been within British Jurisdiction at the time, according to the European Convention on Human Rights.

The high court has reserved its decision and allowed a further week for Rabbat and his lawyers to make additional, specified submissions. If the appeal is not dismissed, it will be up to the supreme court to decide whether or not the crime of aggression exists in English law.