Tony Blair tried to stop the Iraq war inquiry being held in public as new evidence emerged suggesting that he knew Saddam Hussein may not have weapons of mass destruction.

The former Prime Minister lobbied Sir Gus O’Donnell, the head of the Civil Service, fearing that a public appearance at the inquiry, headed by Sir John Chilcott, could turn into a “show trial”.

The move appears to have backfired this weekend, as it emerges that part of the inquiry will now be heard in public and Mr Blair is the focus of calls to appear.

Nick Clegg, the Liberal Democrat leader, said this morning that the former Prime Minister cannot appear behind closed doors and must speak under oath. Otherwise “people [will] feel this is just a grand cover-up for, after all, what was the biggest foreign policy mistake this country has made since Suez”.

Jack Straw, who was Foreign Secretary in the lead up to the Iraq war, said he was more than happy to appear in public, adding “I'm sure [Mr Blair] is too.”

This news comes amid new evidence to suggest that Mr Blair knew that Iraq did not have weapons of mass destruction.

A memo dated January 31, 2003, by Sir David Manning, then Mr Blair’s policy adviser, outlines how President Bush told Mr Blair he had decided on a start date for the war — almost two months before the invasion.

Paraphrasing the President’s comments at the meeting, Sir David noted: “The start date for the military campaign was now pencilled in for March 10. This was when the bombing would begin.”

The memo also suggested that Mr Blair and Mr Bush contemplated other scenarios that might trigger a second UN resolution, legitimising military action in case UN weapons inspectors failed to find WMD.

Mr Bush told Mr Blair that they had developed plans to draw Iraq into combat by flying “U2 reconnaissance aircraft painted in UN colours over Iraq with fighter cover”. If Saddam Hussein fired at the planes, it would put the Iraqi leader in breach of UN resolutions.

In public at this time, Mr Blair was justifying plans for an invasion on the grounds that Iraq might have weapons of mass destruction.

Philippe Sands, a law professor and human rights campaigner, said that the existence of such evidence made it vital that the inquiry was not held in private.

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