Lord Goldsmith expressed his doubts to Prime Minister Tony Blair, Washington's staunchest ally on Iraq, in a document on 7 March 2003, the paper revealed on Wednesday.
Yet two weeks later, a summarised statement read out by the Attorney General to the British Parliament expressed only that Iraq was in likely breach of material sanctions.
Crucially, Lord Goldsmith did not go further in his prepared remarks to Parliament -leaving it in the governments hands to go ahead and link a violation of material breaches by Iraq, with a legal basis to authorise war.
"It was political, not legal grounds to make war on Iraq and Tony Blair is fully responsible for this decision -an illegal decision"
Lawyer Phil Shiner
With Goldsmith doubting the legal validity of this argument, the British government became so worried that it set up a team of lawyers to prepare for any action in an international court challenging Britain's case.
The paper said it based its revealation on a book to be published this week called Lawless World, by law professor and lawyer Philippe Sands of Matrix Chambers -who shares the London offices of the prime minister's barrister wife, Cherie Blair.
According to Sands' book, Goldsmith raised doubts about the legality of military action in advice given to Blair in a 13-page document dated 7 March 2003.
He warned: "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."
According to Phil Shiner of Public Interest Lawyer, a human rights law practice, speaking to Aljazeera net, the crucial bit of Mr Sands revelation was that the statement made by the Attorney General to the British Parliament on 17 March was legally unequivocal, meaning without full legal weight.
"The Attorney General can claim that his legal opinion to Tony Blair on 7 March was legally equivical -a considered opinion that the grounds for war without a second UN resolution was shaky.
"Yet the legal opinion presented to Parliament by Goldsmith [but prepared for him by two lawyers close to Tony Blair] did not make a case to go to war on Iraq, because there was no sound legal basis."
Blair's legal case for war
appears to have been shaky
"Therefore, it was infact on political, not legal grounds to make war and Tony Blair is fully responsible for this decision -an illegal decision," said Shiner, who was named as Britain's human rights lawyer of the year last year by the Bar Council and Law Society.
Since today's news, Goldsmith has not been available for comment. Previously, he has denied claims he swallowed his own doubts about the case for war to give Blair legal cover after London and Washington failed to gain United Nations backing for a resolution authorising military action.
The British government has only published a summary of his advice.
However, Sands says that 10 days later, on 17 March, Goldsmith said in answer to a parliamentary question that it was "plain" that Iraq was in breach of UN resolution 1441 which required it to comply with disarmament obligations.
"Plain to whom?" Sands asks in his book. "[This answer] was neither a summary nor a precis of any of the earlier advices which the attorney-general had provided."
Blair, who is preparing to fight an election expected in May, has refused calls to publish the legal advice Goldsmith gave him.
The war on Iraq dragged down Blair's once sky-high public ratings and divided his ruling Labour Party.