They said Annan would be carrying a document clearly specifying "red lines beyond which Annan cannot go" in talks with Iraqi officials.

CNN: Source: Annan expected to make Baghdad trip

Prime Minister Gordon Brown has resisted calls for a referendum on the treaty, arguing it is substantially different from the constitution document and Britain has "red lines" protecting sovereignty in key areas.

BBC: RELATED BBC SITES

He said that the document highlighted "43 very real and very predictable risks, 21 red-rated, 14 likely to happen with major consequences...emergencies less well-managed, more failures, GP consortia going bust or having to cut services, performance dips and key staff lost".

BBC: Burnham: Government 'took risks' with NHS