An invasion of Iraq was discussed within the Government more than two years before military action was taken – with Foreign Office mandarins warning that an invasion would be illegal, that it would claim "considerable casualties" and could lead to the breakdown of Iraq, The Independent can reveal.

The extent of Whitehall opposition to the policy eventually backed by Tony Blair emerges just three days before Gordon Brown will appear at the Iraq Inquiry, where he will be asked to explain his role in the Government's decision to invade.

Secret Foreign Office strategy papers drawn up by senior civil servants at the end of 2000 have been obtained by this newspaper and are published for the first time today. The Iraq: future strategy document considers options for dealing with the belligerent Iraqi leader Saddam Hussein. It is one of the key documents that Sir John Chilcot's Iraq Inquiry has declined to release.