Baghdad's breach of United Nations resolutions did not make the Iraq war legal, former chief UN weapons inspector Hans Blix said in an interview on Friday.
"I don't buy the argument the war was legalised by the Iraqi violation of earlier resolutions," Blix told Britain's Independent newspaper.
He said it would have required a second UN Security Council resolution explicitly authorising the use of force for the invasion to have been legal.
British Foreign Secretary Jack Straw hit back, saying former Iraqi President Saddam Hussein's repeated violations of UN resolutions justified the conflict.
Saddam had been warned he would face "serious consequences" if he failed to comply with UN resolutions on his weapons programmes. Straw said that meant military action.
"The reason why I believe military action was justified at the time and continues to be justified, was that there was no argument at the time and there really is no argument now but that Saddam was in further material breach of all his (UN) obligations," Straw told BBC Radio.
He said attempts to bring Saddam into line through UN sanctions had proved "profoundly ineffective" for years.
Blix has been highly critical of Washington and London since the war, accusing US President George W. Bush and British Prime Minister Tony Blair of hyping the threat from Iraq.
Since Saddam was toppled, no banned weapons of mass destruction - the reasons Bush and Blair gave for war - have been found.
Blix said while it was possible to argue that Iraq had violated UN resolutions, it was for the UN Security Council to take action, not individual states.
"It's the Security Council that is party to the cease-fire, not the UK and the US individually, and therefore it is the Council that had ownership of the cease-fire in my interpretation," he said.
Blix repeated his belief that Bush and Blair had acted in good faith but said both leaders' credibility had suffered.