Gordon Brown is facing demands to change the rules of the Iraq inquiry this weekend amid fears that the most explosive documents explaining why Britain went to war will not be made public.

As the inquiry enters its second week, the prime minister is under pressure to make key evidence relating to secret government discussions public, including minutes showing how the then attorney general, Lord Goldsmith, changed his mind about the legality of the war.

The demands are made in a letter to Brown from the Liberal Democrat leader, Nick Clegg, who insists that unless the lid is lifted on secrecy, the Chilcot inquiry will fail to satisfy the public's demands for honesty.

Last night the Mail on Sunday claimed Goldsmith wrote to Blair in July 2002, eight months before the war, telling him that deposing Saddam Hussein was a blatant breach of international law. The intention was to make Blair call off the invasion but he ignored the advice, the newspaper says, and banned Goldsmith from attending cabinet meetings. The letter has been handed to the inquiry and both men are expected to be questioned about it in the new year.

The row between Clegg and Brown has escalated the day before Sir David Manning, Tony Blair's former foreign policy adviser, gives evidence to the inquiry about a meeting between George Bush and Blair in January 2003 when they are alleged to have agreed to go to war regardless of whether weapons of mass destruction had been found.

Clegg writes: "The full truth cannot be known unless the presently undisclosed legal advice given to the government by the former attorney general Lord Goldsmith in the lead-up to the war is made public, as well as emails and other communications surrounding his summons to Downing Street on 13 March 2003 which preceded his rewriting of his earlier opinion on the legality of the war." He also demands that Brown allow publication of papers relating to the meeting between Blair and Bush at which Manning was present.

It follows a week when critics of the war, including relatives of the 179 servicemen and women who have died in Iraq, have expressed their anger at the power of civil servants to block vital documents. "Protocols" – measures used to control the release of information – give civil servants and witnesses nine separate grounds on which to block publication of damaging details.

Clegg writes that such powers are draconian for a government that has pledged to allow the truth about Iraq to emerge. "The restrictions on information released by the inquiry are greater than those which apply under current freedom of information rules," he writes.

"Unless you provide Chilcot with the freedom you claimed he has across the floor of the Commons this week, public trust in the final outcome of this review will be deeply damaged."

According to reports, a five-page secret document known as the Manning memo records the White House meeting on 31 January 2003. It allegedly shows that Bush and Blair made a secret deal to carry out an invasion regardless of whether weapons of mass destruction were discovered by UN inspectors. It appears to contradict statements Blair later made to parliament that Iraq would be given a final chance to disarm.

The memo also apparently discloses that Bush floated the idea of letting a spy plane in UN colours fly low over Iraq to provoke Saddam into having it shot down, providing a pretext for invasion.

Under current rules, the inquiry could be prevented from disclosing the Manning memo could be blocked from the inquiry. Other papers can be blocked if they contain commercially sensitive information, which might make it possible for Mr Brown to stopblock the release of papers relating to the procurement of equipment for the armed forces before the war.

That process is widely believed to have cost the lives of British troops who were left without vital kit such as body armour, because the contracts were concluded too late. The revelation comes after a week in which senior civil servants have given damning evidence to the inquiry which appeared to show that Tony Blair misled the public about the Iraq war.