Tony Blair urged Gordon Brown to hold the independent inquiry into the Iraq war in secret because he feared that he would be subjected to a "show trial" if it were opened to the public, the Observer can reveal.

The revelation that the former prime minister - who led Britain to war in March 2003 - had intervened will fuel the anger of MPs, peers, military leaders and former civil servants, who were appalled by Brown's decision last week to order the investigation to be conducted behind closed doors.

Blair, who resisted pressure for a full public inquiry while he was prime minister, appears to have taken a deliberate decision not to express his view in person to Brown because he feared it might leak out.

Instead, messages on the issue were relayed through others to Sir Gus O'Donnell, the cabinet secretary, who conveyed them to the prime minister in the days leading up to the announcement of the inquiry last week.

A Downing Street spokesman last night said: "We have always been clear that we consulted a number of people before announcing the commencement of the inquiry, including former government figures. We are not going to get into the nature of those discussions."

Blair is believed to have been alarmed by the prospect of giving evidence in public and under oath about the use of intelligence and about his numerous private discussions with US President George Bush over plans for war. A spokesman for the former Labour leader would only say last night: "This was a decision for the current prime minister, not for Tony Blair."

The Observer reveals today that six weeks before the war, at a meeting in Washington, the two leaders were forced to contemplate alternative scenarios that might trigger a second UN resolution legitimising military action.

Bush told Blair that the US had drawn up a provocative plan "to fly U-2 reconnaissance aircraft, painted in UN colours, over Iraq with fighter cover". Bush said that if Saddam fired at the planes, he would put Iraq in breach of UN resolutions and legitimise military action.

Last night, Nick Clegg, the Liberal Democrat leader, whose party opposed the war from the outset, said: "If this is true about Blair demanding secrecy, it is outrageous that an inquiry into the biggest foreign policy disaster since Suez is being muzzled to suit the individual needs of the man who took us to war."

Brown provoked uproar in the Commons on Monday when he announced the inquiry's scope, membership and remit. Following protests from military leaders and mandarins, including former cabinet secretary Lord Butler, he announced a partial retreat on Thursday, asking the inquiry chairman, Sir John Chilcot, to consider opening a few sessions to the public.

But the move did not ease pressure for a total climbdown. Last night, Brown appeared cornered as MPs of all parties prepared for a Commons debate on Wednesday in which they look certain to back calls for the inquiry to hold sessions in public "whenever possible".

A Tory motion likely to win wide cross-party backing also calls for the committee to include military experts. The Lib Dems are demanding that it also include constitutional and legal experts to assess the legality of the invasion.

In a sign that the government is preparing to retreat, Chilcot is to meet both Clegg and the Conservative leader, David Cameron, on Tuesday, before the debate. MPs believe that he may then announce a bigger public element to the inquiry in order to avoid the humiliation for Brown of defeat in the Commons.

Chilcot will come under pressure from both leaders to open up the inquiry. Clegg wants a guarantee that witnesses such as Blair will give evidence under oath, while Cameron will ask if the committee can issue an interim report early next year, ahead of a likely spring election.

The Tories say that if Brown does not order a U-turn, an incoming Conservative government will "reserve the right" to widen the scope of the inquiry and increase its powers where necessary after an election.

Sir Christopher Meyer, who was the British ambassador in Washington in the run-up to the war and is likely to be called to give evidence to the inquiry, yesterday backed calls to make it public. "It should be open," he said. "I think it should also have powers of subpoena and people should give evidence on oath. I would be perfectly comfortable with that."

He said the case for openness was increased because there had been "a ton of stuff" published in the US, both via official inquiries and in memoirs written by key players, making public what had previously been confidential. "I would be perfectly happy for the whole embassy archive in Washington [to be disclosed]," he added. "I haven't got a problem with that being made available. Things were very sensitive then, but this is 2009."

On his blog, Alastair Campbell, Blair's former spin doctor, says that "on balance" he believes Brown was right to order the inquiry to be held in private. "I can see the arguments for both sides - openness and transparency favours a public inquiry, but it may well be that the inquiry will do a better job freed from the frenzy of 24-hour media."

In a letter to the Observer, a group of current and former Labour MPs, headed by Alan Simpson, the chairman of Labour Against the War, demands a complete rethink. "Neither the public nor parliament will understand how the prime minister's 'new era of openness' can begin with an Iraq inquiry held behind closed doors," says the letter.