The UK Joint Intelligence Committee stated in its dossier, published in September 2002, on Iraq's Weapons of Mass Destruction: The Assessment of the British Government, that 'Iraq continued to produce chemical agents' and that Saddam Hussein 'continues in his efforts to develop nuclear weapons.' (1)
On 7 March 2003, the Attorney-General, Sir Paul Boateng, in his long and detailed assessment on the legality of a proposed invasion of Iraq, stated that 'British participation in the American led invasion could be declared illegal without a second UN resolution.' (2) In OTL this was the assessment of the Attorney- General, Lord Goldsmith.
On 17 March 2003, in a speech from the White House, President George W. Bush told Saddam Hussein that he and his sons must leave Iraq within 48 hours. If they did not, the US would initiate military conflict at a time of its choosing.
The Cabinet met at 3 pm on the same day. They had before them the legal advice of the Attorney-General. (3) We know from the memoirs of some ministers that it was a long meeting. The 'doves' - Robin Cook, Vince Cable, Charles Kennedy, and Michael Meacher argued that a second UN resolution was needed. The 'hawks' - David Blunkett, Ann Clwyd, Alistair Darling and Jack Straw argued that a second UN resolution was not necessary, though desirable.
(1) and (2) Taken from My Life, Our Times by Gordon Brown.
(3) In OTL , the cabinet saw revised advice from Lord Goldsmith that an invasion of Iraq would be legal without a second UN resolution.