Former British cabinet minister Clare Short is demanding a parliamentary investigation into Attorney General Peter Goldsmith's advice on war with Iraq.
Short said on Thursday night that Lord Goldsmith breached the ministerial code by submitting a summary of his advice to senior ministers.
Two of Prime Minister Tony Blair's key allies were involved in drafting a ministerial answer issued in the peer's name, according to a leak published in the Guardian newspaper.
Short said it was this that had earlier been shown to the cabinet as it contemplated military action. Britain joined the US-led invasion of Iraq on 20 March, 2003.
"It says in the ministerial code that if any advice from the law officers is summarised when it comes to Cabinet the full advice should be attached," she told BBC Radio.
"My view is we need the House of Lords to set up a special committee, summon the attorney, get all the papers out, look at exactly what happened," Short said.
Verdict 'set out'
Goldsmith apparently told an official inquiry the verdict presented to Parliament in his name was "set out" by Blair's ex-flatmate Lord Charles Falconer and top aide Baroness Morgan.
Transcripts of private evidence the peer gave to Lord Butler's investigation into the use of intelligence in the run-up to war were produced by the Guardian.
Britain joined the US-led invasion
of Iraq on 20 March, 2003
The attorney general warned Blair less than two weeks before the invasion that military action could be deemed illegal, according to previous reports.
However, he then met Lord Chancellor Lord Falconer, then a Home Office minister, and the prime minister's director of political relations in 10 Downing Street on 13 March.
Goldsmith reportedly told Butler "they shortly set out my view" in the parliamentary statement on 17 March.
Short, the former international development secretary, who initially pledged not to back war without a second United Nations resolution, said she was initially suspicious but was won over.
Goldsmith denied the Guardian report late on Wednesday, calling it "nonsense" to say that his legal opinion was written by Downing Street insiders.
Blair's official spokesman said he would not comment on leaks.