AFP - The invasion of Iraq in 2003 was illegal, the former chief legal advisor to the foreign ministry told a public inquiry into the war Tuesday, three days before ex-prime minister Tony Blair appears.

"I considered that the use of force against Iraq in March 2003 was contrary to international law," Michael Wood, legal advisor to the Foreign Office between 1999 and 2006, wrote in a submission to the Chilcot inquiry in London.

"In my opinion, that use of force had not been authorised by the Security Council, and had no other legal basis in international law."

The focus of the inquiry has sharpened ahead of the appearance of Blair, who took Britain into the war alongside then US president George W. Bush despite strong opposition at home and abroad.

Blair justified the invasion on Iraqi leader Saddam Hussein's continued defiance of United Nations Security Council resolutions governing his possession of weapons of mass destruction (WMD).

His chief legal advisor, attorney general Peter Goldsmith, gave the green light on the eve of the March 2003 invasion, saying that UN resolution 1441 passed in November 2002 provided a base for military action.

However, many commentators believe Goldsmith was pressured into this view, the war was in fact illegal -- and Blair should be prosecuted accordingly.

Two weeks before the invasion, Goldsmith said it would be preferable to get a second UN resolution explicitly authorising military action, but when attempts to achieve this collapsed, he gave the go-ahead.

Ministers argued that 1441 was an ultimatum stating that Saddam would face serious consequences if he did not cooperate with weapons inspectors. When he became obstructive, the United States and Britain took military action.

However, Wood told the inquiry that 1441 made clear that it was up to the UN Security Council to decide whether Saddam had complied with their demands, not individual states like Britain -- and no such decision had been made.

Wood said he challenged the government's view in January 2003, after then foreign secretary Jack Straw told the US-vice president Dick Cheney that it would be "ok" if no second UN resolution was obtained.

He wrote to Straw: "I hope there is no doubt in anyone's mind that, without a further decision of the council, and absent extraordinary circumstances of which at present there is no sign, the UK cannot lawfully use force against Iraq to ensure compliance with its Security Council WMD resolution."

However, Wood said the minister brushed his concerns aside.

"He (Straw) took the view that I was being very dogmatic and that international law was pretty vague and that he wasn't used to people taking such a firm position," Wood told the inquiry.

The lawyer's testimony is the first time he has expressed his opinion on the war, although his deputy at the time, Elizabeth Wilmshurst, resigned just before the invasion because of her opposition.

In a written submission ahead of her appearance before the inquiry Tuesday, she explained: "I regarded the invasion of Iraq as illegal, and I therefore did not feel able to continue in my post."

Critics of the invasion have argued that Blair should now face prosecution for breach of international law, although experts say this is unlikely.

Philippe Sands, professor of law at University College London, told AFP that Blair may be vulnerable to the international crime of aggression, although this crime is not enforceable in English courts or the International Criminal Court.

"It is a bit of a stretch, but the possibility cannot be excluded," he said, adding: "At the very least Mr Blair may wish to exercise some care in making his international travel arrangements."