Iyad Allawi, the Iraqi interim Prime Minister, will meet Tony Blair for talks in London on Sunday, where he will brush aside comments by Kofi Annan, the UN secretary general, that the war in Iraq was illegal.
Mr Annan also warned that the planned elections for a new Iraqi administration in January would not be credible if the current level of violence continued.
Mr Blair, in giving his support to Mr Allawi, will insist that the elections go ahead to demonstrate that democracy can take root after the conflict.
But last night Mr Blair was condemned by one of his own backbenchers for seeing Mr Allawi. Alan Simpson, vice chairman of the left-wing Campaign Group of Labour MPs, said: "It is offensive that Allawi is coming here. Everyone knows he is Bush's choice. He will not take the country into meaningful democracy. He will take Iraq towards the same war-lordism as Afghanistan."
Cabinet ministers played down Mr Annan's remarks about the legality of the war.
Patricia Hewitt, the Trade and Industry Secretary, said on BBC World Service that the Attorney General, Lord Goldsmith, had made it clear that he believed Britain was acting lawfully. "We spelt out at the time our reasons for believing the conflict in Iraq was indeed lawful and why we believed it was necessary to uphold those UN resolutions," she said.
Mr Annan said: "I have stated clearly that it was not in conformity with the Security Council, with the UN Charter." He added there should have been a second UN resolution before the war, and said: "From our point of view and from the Charter point of view, it was illegal."
Clare Short said the ministerial code of conduct required ministers to uphold international law. "I think [Mr Annan's remarks] show clearly that Tony Blair breached the ministerial code of conduct."
Ms Short, who resigned from the Cabinet, said the Butler report had shed new light on the efforts by Mr Blair to persuade the Attorney General to give legal backing to the war. Lord Butler's inquiry into the intelligence failings over Iraq disclosed that Lord Goldsmith required Mr Blair to put in writing an assurance that it was necessary to go to war to enforce UN resolution 1441. "Tony Blair gave a private assurance to the Attorney General when he was saying that the Attorney had said the war was legal. This was completely unknown to the Cabinet and senior officials across Whitehall at the time," said Ms Short.
There was cross-party criticism in the Commons of the Government over Mr Annan's remarks. Liberal Democrat MPs challenged the leader of the House, Peter Hain, over the legality of the war.
Labour Against the War, which is holding a rally on the opening day of Labour's annual conference in Brighton on 26 September, has called for Britain and the US to be referred to the international court over the legality of the war.