"He has not admitted the policy was a mistake, " a spokeswoman said after an interview the prime minister gave to al Jazeera's new English-language channel prompted an opposition leader to call for an apology over the war.

The interviewer suggested that the West's intervention in Iraq had been "pretty much of a disaster".

Blair replied: "It has, but you see what I say to people is why is it difficult in Iraq?

"It's not difficult because of some accident in planning, it's difficult because there's a deliberate strategy . . . al Qaeda with Sunni insurgents on one hand, Iranian-backed elements with Shia militias on the other . . . to create a situation in which the will of the majority for peace is displaced by the will of the minority for war."

That reply brought a flood of newspaper headlines yesterday like "Iraq invasion a disaster, Blair admits on Arab TV".

Centrist Liberal Democrat leader Menzies Campbell, a long-time opponent of the war, called for an apology.

"At long last the enormity of the decision to take military action against Iraq is being accepted by the prime minister, " he said Meanwhile, Chancellor Gordon Brown flew into Iraq for the first time yesterday and pledged an extra £100m to help rebuild the country.

Brown was on a surprise visit to troops in southern Iraq and trying to boost his credentials to take over from prime minister Tony Blair sometime over the next year. "What I'm saying today is we could provide an extra £100m over the next three years to help with the economic regeneration programme, " Brown told reporters.

Travelling with Britain's chief of the armed forces Sir Jock Stirrup, Brown will meet Iraqi deputy prime minister Barham Salih and other senior officials on his visit to the region.

He said: "We are committed to supporting the Iraqis in building a democratic nation which brings security and prosperity to its people and plays a full part in the region and the world economy."

Britain was part of the US-led invasion of Iraq in 2003 to topple Saddam Hussain. It has 7,200 troops in southern Iraq, mostly stationed in and around Basra. But the city remains dangerous with Shi'ite factions battling each other for control and British troops occasionally caught in the middle.

The visit is the latest initiative by Brown to widen his brief beyond his treasury portfolio as he looks closer than ever to succeeding Blair.

Bowing to pressure from within his own Labour partly because of the unpopularity of the Iraq war, Blair has said he will leave office within a year but he is widely expected to go around May.