He (Anan) has carefully avoided the word (illegal) before.
His previous phrasing was to say that the war was "not in conformity with the UN Charter".
...
BBC: "So you don't think there was legal authority for the war."
Mr Annan: "I have made it clear, I have stated clearly, that it was not in conformity with the UN Charter."
BBC: "It was illegal."
Mr Annan: "Yes, if you wish."
BBC: "It was illegal."
Mr Annan: "Yes, I've indicated that it was not in conformity with the UN Charter. From our point of view, from the charter point of view, it was illegal."
...
So why does he think it illegal?
In the interview, he remarked that Resolution 1441, passed on 8 November 2002, warned Iraq that there would be "serious consequences" if it did not comply with UN demands over its suspected weapons programmes.
Mr Annan said it should have been left to the UN Security Council, in a second resolution, to determine what those consequences were.
The United States and Britain argued they were carrying out the wishes of the Security Council and that their authority was based not just on Resolution 1441, but on previous UN resolutions.
The British Attorney General, Lord Goldsmith, issued a public statement of his outline argument, though he did not publish, or even give the members of the cabinet, his detailed reasoning.
He said the original Resolution 678 from 1990, which allowed for "all necessary means" to end Iraq's occupation of Kuwait and "restore international peace and security" in the region, still applied.
It had been, he said, "revived" by Resolution 687 from 1991, which demanded that Iraq disarm. Since Resolution 1441 stated that Iraq was in breach of Resolution 687, the attorney general argued, there was authority to use force.