Coalition forces failed to plan properly for Iraq's insurgency after the ousting of Saddam Hussein, a committee of MPs says in a report.
Falluja was the focus of intense fighting with insurgents
Planning for the post-conflict phase in Iraq was "marred by a series of mistakes and misjudgements", the Commons Defence Committee said.
The coalition also underestimated the reconstruction needed, it said.
Although there had been "encouraging signs" recently, UK troops would need to remain in Iraq until 2006, it added.
This was because Iraq's security forces would not be capable of taking over until then at the earliest.
It added that because of the security situation, Britain's deployment of 8,000 troops was unlikely to be reduced until 2006 - almost three years on from the toppling of Saddam Hussein.
Foreign Secretary Jack Straw has already said British forces are likely to stay in Iraq until the end of 2005 - after this date their mandate would have to be renewed by the Iraqi government.
The committee said it believed Iraq's rulers would want Britain to stay on until their own security forces were in a position to take over.
"This may be a substantial period of time. We support this commitment and believe that calls for a withdrawal of British forces are premature," the MPs said.
"Experience has taught us that if nation-building exercises, such as that in Iraq, are to succeed, they must have a serious commitment of time, energy, financial resources and political resolve."
Responding to the report, Downing Street said it was always looking at troop levels but the aim was to withdraw when it was felt Iraqi forces were able to deal with the situation.
However, the committee said the coalition should have foreseen the insurgency that followed the defeat of Saddam's regime.
It also failed to foresee the influx of foreign fighters and should have realised that their presence would be resented as "cultural and economic" imperialism, the MPs added.
Iraq's security sector should have been subject to reform immediately after the coalition's victory, they said.
"Only belatedly did the coalition begin building the Iraqi security forces," but even then its approach was characterised by a "short-termism and indecision".
Weaknesses in the reforms "came close to undermining the success of initial military operations," the committee concluded.
But the committee did point to "considerable success" in Iraq, especially in areas controlled by British forces.
Committee chairman Bruce George said any progress made had come about "largely through the professionalism, flexibility and pragmatism of British Armed Forces".
Iraqi police have frequently been targeted by insurgents
Shadow defence secretary Nicholas Soames said he had "consistently warned" the government about the dangers of a coherent plan for post-conflict Iraq.
"This serious failure clearly set the reconstruction of Iraq back by at least a year and resulted in further suffering for the people of Iraq."
Liberal Democrat defence spokesman Paul Keetch said the report endorsed his party's view that the aftermath would be more difficult to deal with than the military action itself.
"What coalition forces failed to do was to prepare for the aftermath," he said.
He said it was a mistake to disband the Iraqi army because it was "the only organisation that could have helped us".