Blix told the London-based Independent daily that a second United Nations resolution explicitly authorising the use of force would have been required to make the invasion of Iraq last March lawful.

"I don't buy the argument the war was legalised by the Iraqi violation of earlier resolutions," Blix said.

Blix attacked reasoning by Attorney General Peter Goldsmith, the British government's top legal adviser, that UN resolution 1441 authorised the use of force because it revived earlier resolutions passed after the first Gulf war.

Blix said that while it was possible to argue that Iraq had violated UN resolutions adopted since 1991, the "ownership" of the resolutions rested with the entire 15-member UN Security Council and not with individual states.

"It's the Security Council that is party to the ceasefire (after the first Gulf war), not the UK and US individually, and therefore it is the Council that has ownership of the ceasefire, in my interpretation."

Asked whether in his view a second resolution authorising force should have been adopted, Blix replied: "Oh yes."

He dismissed calls for British Prime Minister Blair to resign or apologise over the failure to find weapons of mass destruction in Iraq, cited by London as the main justification for war.

But Blix suggested that Blair may have been wounded politically.

"Some people say (US President George W.) Bush and Blair should be put before a tribunal and I say that you have the punishment in the political field here," Blix told the Independent.

"Their credibility has been affected by this: Bush too lost some credibility."