Gordon Brown is to promise that much of the Iraq inquiry will be held in the open in an attempt to avert a damaging Commons defeat for the Government this Wednesday.

Amid fury on the Labour back benches over Mr Brown's initial decision to stage the inquiry in private, ministers now expect much of the evidence to be given publicly after a change of heart was forced on the Prime Minister.

The Labour rebels' anger was intensified by the disclosure yesterday that Tony Blair, likely to be the key witness, had consulted with the Cabinet Secretary on the form of the inquiry. They want him to give evidence under oath.

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Mr Blair's involvement in discussions with Sir Gus O'Donnell over the nature of the hearings was confirmed by Shaun Woodward, the Northern Ireland Secretary. "Of course the Cabinet Secretary discussed this with the former prime minister," Mr Woodward said, "because he obviously will be one of the major witnesses who will be giving evidence to Sir John Chilcot's inquiry".

The backbenchers also pointed to a leaked memo yesterday indicating that the former prime minister had been considering the possibility of going to war without a second UN resolution two months before the invasion.

The note, written by his foreign policy adviser Sir David Manning, indicated that Mr Blair and US President George Bush were already discussing ways of legitimising military invasion in case the UN failed to find weapons of mass destruction.

Such documents are likely to go to the heart of the inquiry; suggestions they could be examined in secret provoked uproar among MPs of all parties and senior military and intelligence officers. Mr Brown has already staged a partial retreat by asking Sir John Chilcot, the retired civil servant who will head the inquiry, to hold some sessions in public. But the concession did not go far enough to pacify Labour MPs threatening to support a Tory motion on Wednesday calling for all hearings to be held in public other than for security reasons.

Sir John will tomorrow meet David Cameron, the Tory leader, and Nick Clegg, the Liberal Democrat leader, to discuss the form of the inquiry.

Ministers said yesterday that they expected the bulk of hearings to be held in public and one senior Whitehall source said: "It's inevitable that will happen."

Jack Straw, foreign secretary at the time of the war, said the indications from Sir John were that his hearings would be both public and private. He said: "I have no problem with giving most of the evidence I have in public."

Mr Straw told BBC1's The Andrew Marr Show that he was sure Mr Blair would also be happy to appear publicly.

Sadiq Khan, Britain's most senior Muslim politician, admitted the controversy over the hearings "looked awful", blaming the furore on ignorance over the autonomy given to the chairs of inquiries. Mr Khan, a Transport minister, told BBC1's Politics Show: "I suspect there will be many, many parts of the inquiry held in public."

The Government is preparing to table a rival motion on Wednesday promising widespread public hearings in an effort to peel off MPs reluctant to support a Conservative motion. Last night Labour MPs opposed to the war said they would only be satisfied by the majority of hearings being public.

Paul Flynn, the MP for Newport West, said: "We want a clear assurance that the inquiry will be open." Gordon Prentice, MP for Pendle, argued: "The whole way this has been done is so cack-handed and inept it is unbelievable.

"The inquiry should be open with evidence given on oath. There must be an opportunity for the leading players to be cross-examined."