British Foreign Secretary David Miliband PHILIP HOLLIS FOR TIME

Mr Miliband made the admission as anti-war campaigners used the fifth anniversary of the start of the invasion of Iraq to repeat calls for British troops to be brought home.

Mr Miliband told GMTV: "I think the war itself was a remarkable victory. It went better than most people expected. But the truth is that building the peace after the war has been much more difficult than people expected."

He continued: "The truth is, it's been very tough to help build a more stable society in Iraq but I think the indications over the last year or two have been more encouraging about the changes."

He said: "Eleven million Iraqis did vote for the government they've got and we're there to try to defend the integrity of that government and to help it run a decent country.

"Yes, it's a very dangerous situation for our troops and for the Iraqis, but I think it is striking that the number of attacks is down, the amount of trade that is going on is up, the economic situation has improved, and that, in a way, is a function of the security improvement."

The anti-war protest came as an audio message apparently from Osama bin Laden warned Europeans of a "severe" reaction to come, condemning the publication of drawings insulting to the Prophet Mohammed.

Campaigners including the Stop The War Coalition (STWC), CND and the British Muslim Initiative staged a vigil outside Downing Street and delivered a letter to Prime Minister Gordon Brown demanding a public inquiry into the conflict, with access to all Government papers and minutes of Cabinet meetings as well as the power to question former premier Tony Blair.

The letter to Mr Brown described the Iraq war as "disastrous" and called for March 20 to be declared International Peace Day.