The paper also said that Elizabeth Wilmshurst, deputy legal adviser at the Foreign Office, who resigned in protest against the Iraq war, described the planned invasion as a “crime of aggression”.

She also said that she could never agree to military action in circumstances she said was "so detrimental to the international order and the rule of law".

The Guardian said that its report was based on a book to be published this week called “Lawless World," written by law professor and lawyer Philippe Sands, who shares the London offices of Blair’s wife, Cherie Booth.

"So concerned was the government about the possibility of such a case that it took steps to put together a legal team to prepare for possible international litigation," Sands wrote.

Goldsmith couldn’t be reached for comment.

The attorney general has previously denied claims that he swallowed his own concerns about the Iraq war to provide Blair with a legal cover after the UK and the U.S. failed to win the United Nations support for a resolution authorizing military action.

According to Sands, Goldsmith warned that: "If the argument were to come before a court of law it might well be unsuccessful, so the use of force could be found to be illegal."

But Sands said that ten days later, on March 17, Goldsmith said in parliament that it was “plain” that Iraq was violating UN resolution 1441 which demanded it to comply with disarmament obligations.

"Plain to whom?" Sands asks in his book. "(Goldsmith remarks) was neither a summary nor a precis of any of the earlier advices which the attorney general had provided."

Blair, who is getting ready to fight an election expected in May, has rejected calls to release Goldsmith’s legal advice.

The invasion of Iraq has affected the prime minister’s public ratings and divided his own ruling Labour Party.