The UK's role in the war remains highly controversial
Britain may have had "second thoughts" about the Iraq invasion had the scale of post-war violence been anticipated, the Iraq inquiry has heard.
Sir John Sawers, now head of MI6, said "very few observers" foresaw that Iraq would attract al-Qaeda terrorists and Shia extremists backed by Iran.
Only Egyptian president Hosni Mubarak got it right when he warned it would unleash "100 Bin Ladens".
The inquiry is examining UK policy towards Iraq between 2001 and 2009.
Sir John, the UK's special envoy to Iraq from May to July 2003, told the inquiry: "It was unprecedented, the scale of violence that we faced in Iraq."
AT THE INQUIRY
BBC World Affairs correspondent Peter Biles
With the steady accumulation of evidence over the past few weeks, there has been a noticeable and welcome change of tone at the inquiry.
Sir John Chilcot's five-member committee now has some important issues to raise with the key witnesses. A more robust line was taken today as top Whitehall civil servants were asked to assess the cost of the Iraq War.
The recurring theme has been the extent to which everyone underestimated the scale of the violence in post-war Iraq. It was so bad that MI6 chief, Sir John Sawers, was forced to admit that Britain might have had "second thoughts" about getting involved, had it been possible to predict the violence.
Sir John argued that Iraq is "a better place" than it was before the 2003 invasion, and there has been "no sustained damage" to Britain's reputation in the Islamic world.
But six years on, it is clear there was a desperate need for better planning, improved understanding of Iraq and a more joined-up approach, both in Whitehall, and between London and Washington.
He said the mindset of the Americans was that post-invasion Iraq would be like occupied Germany in 1945.
"We all have an image of that, a sullen population defeated but no sustained violence against the victorious forces.
"Very few observers actually highlighted the scale of the violence that we could face. I think about the only person in my recollection who got it right was President Mubarak who warned of unleashing 100 Bin Ladens."
He said undefeated elements of the Baathist regime combined with international terrorists and Iran-backed "Shia extremists" had created an "onslaught of violence that was not thought through by any observer".
Sir John said: "I think frankly had we known the scale of violence, it might well have led to second thoughts about the entire project and we could certainly have mitigated some aspects of it had we had a clearer appreciation of it in advance."
But he said it was not "reasonable" to assume the violence should have been predicted as it was an "unprecedented scenario".
He said within months of the 2003 invasion, British officials were aware there were "difficulties" in the way Iraqi detainees were being treated but thought they were limited to poor conditions and "possibly unnecessary violence".
But he said Abu Ghraib, the prison which in 2004 became notorious for the abuse of detainees by US forces, was "way beyond anything that we envisaged".
"The revelations at Abu Ghraib were definitely a shock to us," he added. Spring 2004 had been a low point, Sir John said when the insurgency and violence was worsening and Abu Ghraib "added another nasty twist".
Inquiry panel member Sir Roderic Lyne also asked whether the Iraq war had been worth the "very high cost".
Sir Nigel Sheinwald, who was then PM Tony Blair's foreign policy adviser from 2003 to 2007, said: "That is a very difficult decision and a very difficult debate. I can't answer it even now."
November-December: Former top civil servants, spy chiefs, diplomats and military commanders to give evidence
January-February 2010: Tony Blair, Gordon Brown and other politicians expected to appear before the panel
March 2010: Inquiry expected to adjourn ahead of the general election campaign
July-August 2010: Inquiry expected to resume
Report set to be published in late 2010 or early 2011
Sir John said he did not believe the war had led to lasting damage of Britain's reputation in Muslim countries and added: "The Iraqis undoubtedly have benefited enormously from the sacrifice of the 179 British personnel who died in the Iraq conflict between 2003 and 2009."
Earlier Lt Gen Sir Robert Fry, deputy chief of the defence staff in the run-up to the war, said the UK's role in the March 2003 invasion was crucial.
Asked what the UK stood to lose if it did not contribute troops to the Iraq invasion once planning was advanced, Sir Robert said "maybe the war".
Panel member Sir Roderic remarked that the commander was the first witness to suggest that the UK's contribution was "critical" to winning the war.
Gen Fry replied: "Let me put it a different way. The US scheme of manoeuvre had the potential of failure within it."
Other witnesses have suggested that the UK could have sent fewer troops to Iraq and only chose to send the maximum number to bolster its relationship with Washington.
Sir Robert also said divisions in government, cabinet and public opinion over the legitimacy of the war inhibited post-war reconstruction efforts.
Sir Robert criticised the role of the Department for International Development (DFiD), saying some of its officials "could barely conceal their moral disdain" for the military action.
Senior politicians, including Tony Blair, are to appear before the inquiry next year, with its report due to be published in late 2010 or early 2011.