Straw told Blair of Iraq legal problems

Jack Straw, the Justice Secretary, secretly warned Tony Blair that there were major legal obstacles to invading Iraq, it has been revealed.

In a private letter to the former Prime Minister, Mr Straw also said that Britain needed to give serious thought to what would happen after Saddam Hussein was removed, as daily conditions for ordinary Iraqis may not improve.

The correspondence, which was sent a year before the 2003 invasion, appears to show that military action was on the Government's agenda long before troops entered Iraq. It is expected to form a central part of Mr Straw's evidence to the Iraq inquiry when he appears before Sir John Chilcot's team this week.

Outlining the legal "elephant traps" facing any invasion, Mr Straw warned Mr Blair that "regime change per se is no justification for military action" and "the weight of legal advice here is that a fresh [UN] mandate may well be required". No such resolution giving the go-head for an invasion was secured.

The inquiry is set for its most riveting week yet, as three key figures appear to give evidence. Today it will hear from Jonathan Powell, Mr Blair's head of staff, who is expected to be asked about his involvement in the notorious September 2002 dossier setting out the case for war in Iraq. Former Defence Secretary, Geoff Hoon, will appear tomorrow, while Mr Straw will give evidence on Thursday.

Mr Hoon faces questions over allegations that he prevented the military from ordering vital equipment for British troops. Lord Boyce told the inquiry last year that Mr Hoon delayed him from making the arrangements because he was afraid the military preparations would become public.