The government did not fully consider the implications of the Iraq invasion, a former senior aide to Tony Blair has admitted.
We prepared for the "wrong kind of aftermath", says Jonathan Powell.
Ex-Downing Street chief of staff Jonathan Powell said the preparation was for the "wrong kind of aftermath".
He said on BBC1's Andrew Marr show: "We probably hadn't thought through the magnitude of what we were taking on."
Meanwhile, the Tories have called for a Privy Council inquiry into the "origins and conduct" of the Iraq war.
Mr Powell told BBC1's Andrew Marr show: "We made lots of preparations for humanitarian disaster, for the lack of water and that kind of thing.
"What we hadn't, in my view, really thought through was the long-term nature of this."
He added: "I think we probably hadn't thought through the magnitude of what we were taking on in Iraq. This is something that will take many decades to sort out."
Also speaking on the Andrew Marr show, shadow foreign secretary William Hague said the fifth anniversary of the war was the right time for a full inquiry.
"Clarity" was needed on the future for Britain's deployment in Iraq, he said.
And he demanded: "It is very important to commence the full-scale Privy Council inquiry into the origins and conduct of the war, because if we are not going to start it now, five years on from the beginning of the war, then when on earth would we have such an inquiry?
"Lessons have got to be learned, and visibly learned, and we have got to start on that process now. We will be raising this again in Parliament in the coming weeks."
There have been four inquiries into different aspects of the Iraq war, including the Butler report into intelligence failings and the Hutton inquiry.
But there has not been an inquiry focusing on how the government's decision to join the US-led invasion was made.
William Hague wants a comprehensive inquiry to start now
The government has indicated it will sanction a high-level inquiry into the war once British troops had been withdrawn.
Groups opposed to the continuing war in Iraq are marking the fifth anniversary of the Iraq invasion with demonstrations.
Thousands of protesters marched in London and Glasgow on Saturday ahead of the March 20 anniversary to demand Britain withdraw its troops from both Iraq and Afghanistan.
And the Liberal Democrats have repeated their demands for an "early and full" withdrawal of British troops.
The party's leader Nick Clegg said Iraq was the biggest strategic failure of British foreign policy for decades.
Mr Clegg said the date should remind government and Tories not to "blindly" follow Washington's lead.
"On this anniversary of the greatest strategic failure in British foreign policy in the post-war period, both Labour and the Conservatives must learn from their fateful decision to back George Bush's invasion of Iraq," he said.
"It is now time to recast British foreign policy in Britain's interests, not those of Washington, starting with an early and full withdrawal of all remaining British troops in Iraq."