He insisted he gave serious attention to a warning from his former senior legal adviser, Sir Michael Wood, that the conflict would be a 'crime of aggression' unless Britain achieved another UN Security Council resolution.
The Chilcot Inquiry into the war has heard that Mr Straw, who was foreign secretary at the time of the 2003 invasion, rejected Sir Michael's advice.
But Mr Straw said in a statement to the inquiry today that it would be a 'fundamentally flawed' system if ministers were obliged to accept all the legal advice they received.
He wrote: 'Far from 'ignoring' this advice, as has been suggested publicly, I read Sir Michael's minute with great care and gave it the serious attention it deserved.
'So much so that I thought I owed him a formal and personal written response rather than simply having a conversation with him.'
