Former attorney general Lord Goldsmith defended his legal advice
Legal advice given to Tony Blair by the attorney general prior to the Iraq war was fundamentally "flawed," a former law lord has claimed.
Lord Bingham said Lord Goldsmith had given Mr Blair "no hard evidence" that Iraq had defied UN resolutions "in a manner justifying resort to force".
Therefore, the action by the UK and US was "a serious violation of international law," Lord Bingham added.
Lord Goldsmith said he stood by his advice to the then prime minister.
The Liberal Democrats say that Lord Bingham's comments made a full public inquiry "unavoidable" into the decision to invade Iraq.
Responding to Lord Bingham's criticism, Lord Goldsmith insisted the invasion of Iraq was legal.
"I would not have given that advice if it were not genuinely my view," he said.
Lord Bingham, a former Lord Chief Justice, made his comments in a speech on the rule of law at the British Institute of International and Comparative Law in London.
He referred to a written parliamentary statement made by Lord Goldsmith on 17 March 2003 in which he confirmed that war on Iraq would be legal on the grounds of existing UN resolutions.
Lord Bingham said: "This statement was flawed in two fundamental respects.
"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 also criticised Lord Goldsmith for failing to make clear that only the UN Security Council could judge whether there had been compliance and, if appropriate, authorise further action.
"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," he said.
Lord Goldsmith said his critic was "entitled to his own legal perspective".
"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 said.
He also said the UN resolution that Iraq was deemed to have defied - 1441 - did not need further determination by the Security Council.
Lord Chancellor Jack Straw backed Lord Goldsmith, arguing that his advice "was shared by many member states across the world".
"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," Mr Straw said.
But Lib Dem leader Nick Clegg said Lord Bingham's claims made a full public inquiry into the government's decision to go to war "unavoidable".
"Lord Bingham's stature means that his devastating criticism cannot just be brushed under the carpet," Mr Clegg said.
"This is a damning condemnation of what was an unjustified invasion which we now know to have flouted international law."
Former lord chief justice Lord Bingham retired from the bench in July. Lord Goldsmith stepped down from his post as attorney general last year.