Dr Rowan Williams, the Archbishop of Canterbury, today launched a powerful and unprecedented attack on the Government over its policy in Iraq.

He accused Tony Blair and his ministers of rushing into the war, of not being sufficiently truthful and of creating "a weakening of trust in the political system of our nation".

Arguing that trust could be restored by the admission of error or miscalculation, Dr Williams even made a case for civil disobedience, arguing that political obedience in our age has become "problematic". Anglican theologians have never sanctioned compliance with "unjust law", he said.

Dr Williams was preaching the university sermon at St Benet’s Church, Cambridge. The sermon was endowed by John Mere, a fellow of Corpus Christi, more than four centuries ago to teach "due obedyence of the subjectes to their princyes".

Dr Williams has often made clear that, along with with nearly all religious leaders in Britain, he was opposed to the war in Iraq, but he has never before been openly critical of the present Government on this or any other policy.

Invited two years ago to deliver the Mere Commemoration sermon, he has been developing his argument for some months in the light of the events of last year.

While appearing to advocate civil disobedience, sources close to him made clear that this should only occur in the most extreme circumstances and the UK was not yet at that point.

The example he used in his sermon was of the American civil rights movement. Dr Williams said that Christian political obedience must rest on confidence in a government's capacity for attention.

He condemned the "idle and selfish hearts" of those who advocate Christian obedience while themselves being slow to bring their own thoughts "under obedience to Christ".

Dr Williams said: "Part of the continuing damage to our political health in this country has to do with a sense of the events of the last year on the international scene being driven by something other than attention.

"There were things government believed it knew and claimed to know on a privileged basis which, it emerged, were anything but certain. There were things which regional experts and others knew which seemed not to have received attention."

Dr Williams, a former lecturer in divinity at Cambridge, addressing a congregation of academics, clerics and students, continued: "Forgetting the melodramatic language of public deception, which is often just another means of not attending to what is difficult and takes time to fathom, the evidence suggests to many that obedience to a complex truth suffered from a sense of urgency that made attention harder."

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