Legal advice given to Tony Blair prior to the invasion of Iraq was fundamentally "flawed", a former senior law lord said today.
Lord Bingham, who stepped down in July, described the action by the UK and US as a "serious violation of international law".
He said advice by the then Attorney General Lord Goldsmith failed to acknowledge the lack of hard evidence implicating Iraq's non-compliance with UN resolutions. Furthermore it neglected to make clear that only the Security Council could authorise further action, Lord Bingham suggested.
Lord Goldsmith said tonight that he stood by his advice to the then prime minister.
Lord Bingham's comments came in a speech on the rule of law at the British Institute of International and Comparative Law.
In it he referred to a statement made by the former attorney general in March 2003 as Britain prepared for the invasion.
"This statement was flawed in two fundamental respects," he said.
"It was not plain that Iraq had failed to comply in a manner justifying resort to force and there were no strong factual grounds or hard evidence to show that it had.
"Hans Blix and his team of weapons inspectors had found no weapons of mass destruction, were making progress and expected to complete their task in a matter of months."
Lord Bingham added that Lord Goldsmith's advice passed on the belief that a decision over whether Iraq had not complied with UN resolutions could be made by anyone other than the UN Security Council.
He said: "If I am right that the invasion of Iraq by the US, the UK and some other states was unauthorised by the Security Council there was, of course, a serious violation of international law and of the rule of law."
Responding to the criticism, Lord Goldsmith said: "I stand by my advice of March 2003 that it was legal for Britain to take military action in Iraq.
"I would not have given that advice if it were not genuinely my view. Lord Bingham is entitled to his own legal perspective five years after the event, but at the time and since then many nations other than ours took part in the action and did so believing that they were acting lawfully."
He added that the UN resolution that Iraq was deemed to have failed to comply with, and which ultimately led to the invasion, did not need further determination by the Security Council.
Lord Chancellor Jack Straw said: "I endorse what Lord Goldsmith has said. Lord Goldsmith's advice that military action was lawful and in accordance with Security Council Resolutions was shared by many member states across the world.
"Put simply, the central issue during the negotiations of 1441 was over the requirement for a so-called second resolution, and whether or not an additional lock requiring a further decision on military action should be included in 1441.
"In the event, the agreed text of 1441 was in accordance with the US/UK position that this resolution did not require this further lock.
"So I do not accept Lord Bingham's conclusions, which do not, I am afraid, take proper account of the text of Security Council Resolution 1441 nor its negotiating history."