US President George W Bush has announced an inquiry into pre-war intelligence that said Iraq possessed weapons of mass destruction.
No weapons of mass destruction have been found in Iraq
He told reporters he would set up an "independent, bipartisan" panel.
Mr Bush will appoint the members and set the terms of the commission, which is not expected to report before the presidential election in November.
The UK Government is also poised to hold an inquiry - the details of which may be announced to MPs on Tuesday.
The prime minister's official spokesman said a statement would be made to MPs on an inquiry seeking to answer "valid questions" about the failure to find weapons of mass destruction (WMD).
It had been suggested a formal announcement could take place on Monday although later that was ruled out, prompting speculation there would be confirmation on Tuesday.
The main argument used by Britain and the US for invading Iraq last March was the perceived threat from weapons of mass destruction. But no such weapons have yet been found.
I am putting together an independent, bipartisan commission to
analyse where we stand, what we can do better as we fight this war
Announcing the US inquiry, Mr Bush said: "I want to know all the facts".
But he insisted that former Iraqi leader Saddam Hussein had been a danger.
"We know he had the intent and capabilities to cause great harm," Mr Bush said.
Mr Bush said he planned to consult former US chief weapons inspector David Kay, who has said that Iraq did not have stockpiles of banned weapons, before creating the panel.
Pressure has been growing for an independent inquiry since Mr Kay told Congress last week that "it turns out we were all wrong, probably", about the threat from Iraq.
Mr Bush said the inquiry would look into how US intelligence has handled issues of proliferation and WMD, as well as the way security services obtained information about secretive regimes such as North Korea and Iran, and groups like al-Qaeda.
White House sources say the inquiry will be modelled on the Warren Commission, which investigated President Kennedy's assassination in the 1960s. The panel is expected to have nine members, both Republicans and Democrats.
WMD INTELLIGENCE STATEMENTS
28 Jan: David Kay tells Congress: "We were almost all wrong" in assuming Iraq had illicit weapons
29 Jan: National Security Adviser Condoleezza Rice acknowledges flaws in pre-war intelligence
30 Jan: President Bush says he wants the "facts"
2 Feb: President Bush says he will order an inquiry
Observers suggest that the Central Intelligence Agency and its director, George Tenet, may face heavy criticism.
But former CIA director James Wolsey said Saddam Hussein may have been involved in an elaborate deception, telling even his own military chiefs that Iraq had WMD.
"If Saddam was deceiving his own generals, even if we had recruited a dozen Iraqi generals as American spies we still would have gotten a false story," he told the BBC's World Today programme.
The decision to hold inquiries in the US and UK will increase pressure for a similar inquiry in Australia, which also joined the Iraq war.
Australian Prime Minister John Howard has said he will take into account Mr Bush's decision to hold an inquiry. But he added that most of Australia's intelligence on Iraq came from the US and Britain.
In Spain, opposition socialists are calling for the government there to order an inquiry into the intelligence used to justify the war. Spain was a key supporter of the Iraq war, although it did not commit troops to battle.