Kofi Annan also told The Times (£) that the former Prime Minister failed to live up to early expectations during his time in power to the point in 2006, when he could no longer "act as a credible mediator” in international affairs.
He told the newspaper:
Blair had the potential to be one of the most brilliant politicians of his time and really for a period was a star. And now you ask me the questions, ‘What went wrong? What changed him?’ It is very difficult to say.
Tony Blair was the only man with the power to stop President Bush from invading Iraq in 2003, former UN Secretary-General Kofi Annan has claimed.
Mr Annan said unlike himself and then US Secretary of State Colin Powell, Mr Blair had the ear of President Bush ahead of the war and could have acted.
Speaking to The Times (£), he said: "I think I will for ever wonder what would have happened if, without a second [UN] resolution ... Blair had said 'George, this is where we part company. You’re on your own."
Mr Annan added: "I really think it could have stopped the war ... It would have given the Americans a pause. It would have given them a very serious pause to think it through ... All this would have raised a question: ‘Do we go this alone?’"
International peace envoy Kofi Annan has arrived in the Syrian capital of Damascus for talks with President Bashar al-Assad, his spokesman said.
It comes a day after the international peace envoy admitted that his peace plan had so far failed to end 16 months of bloodshed.
UN and government sources said Annan and Assad would not meet tonight.
The UN envoy to Syria Kofi Annan has said he is "optimistic" that the meeting of world leaders in Geneva on Saturday will produce an acceptable outcome.
Mr Annan's spokesman Ahmad Fawzi told Reuters TV that "talks are on course" and a preparatory meeting will be held in Geneva on Friday.
Kofi Annan said he was "frustrated and impatient" over the continued violence and killing in Syria, and said he wanted to see faster progress towards resolving the crisis.
Following talks in Beirut with Lebanon's Prime Minister Najib Mikati, he said:
I think perhaps I am more frustrated than most of you because I am in the thick of this. I want to see things move faster.
Countries from Australia to the UK have expelled Syrian diplomats in a coordinated move to step up pressure against the regime.Read the full story ›
International mediator Kofi Annan said on Monday he was horrified by the killings in the Syrian town of Houla.
He urged the Syrian government to take bold steps to show it was serious about reaching a peaceful solution to the country's crisis.
Speaking shortly after arriving in Damascus for talks with the government, Annan said he expected to have "serious and frank discussions" with Syrian President Bashar al-Assad.
UN Secretary-General Kofi Annan has warned the UN Council today that if the world did not act then Syria will go into a "full-scale civil war". Mr Annan inferred that the day may come to take a "different tack" than his peace plan.
UN Secretary-General Kofi Annan has said today that there has been a "decrease in military activities" in Syria, but the level of violence is still "unacceptable".
Mr Annan told a conference in Geneva that the UN could not allow Syria to go into "full-scale civil war", but the latest spate of bombings were "worrying" sign. He said:
"If it fails and it were to lead into a civil war, it will not affect only Syria, it will have an impact on the whole region.
A spokesman for the UN special envoy Kofi Annan says the peace plan for Syria is "on track", after signs of compliance, although slow and small.