Former UN secretary general says in memoirs he admired ex-British PM until he failed to stand up to US over invasion

Tony Blair could have stopped the Iraq war had he decided to walk away from a partnership with the US, the former UN secretary general Kofi Annan has claimed.

In an interview to launch his memoirs, Annan said he had reflected on what would have happened if, without a second UN resolution over Iraq, Blair had refused to go to war with Iraq in 2003.

"I will forever wonder what would have happened if, without a second [UN] resolution ... Blair had said 'George [Bush], this is where we part company. You're on your own'," he told the Times. "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?'"

While Annan argued that neither his resignation as UN secretary general or that of then US secretary of state, Colin Powell, would have changed the course of military action, Blair could have made a difference had he spoken out. "Because of the special relationship and also the fact that ... when you think of the big countries, Britain was the only one that teamed up with [Bush]," Annan said.

Annan said he had done everything in his power to stop the war, which was justified heavily on the case that Iraq's ruler Saddam Hussein possessed weapons of mass destruction. No evidence of these were later found and Blair came under fire for relying on a "sexed-up" dossier, which claimed that Hussein had the capacity to activate biological and chemical weapons in 45 minutes. The US-led invasion led to a war that lasted for eight years and is believed to have cost more than 100,000 lives.

"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," he said.

Anann's disappointment in Blair is also reflected in his memoirs, Interventions – A Life in War and Peace, co-written with Nader Mousavizadeh, Annan's former special assistant at the UN. Annan recounts a meeting with Blair in 2006 over a bloody conflict between Israel and the Shia Islamic militant group Hezbollah, which he said the former British prime minister saw simplistically – like Iraq – "a meta-conflict between modernity and the medieval, between tolerant secularism and radical Islam".

"This was not the Blair with whom I had agreed so passionately about the moral necessity of a humanitarian intervention to halt the Serbian attacks on the Kosovar Albanians in 1999 ... Something had changed in Blair, and with it, I felt, his ability to act as a credible mediator," he said.

However, Annan said he did not agree with fellow Nobel peace prize laureate, Archbishop Desmond Tutu, over his suggestion that Bush and Blair should face proceedings in the international criminal court. Annan said they were democratically elected leaders acting in what they believed were their national interests.

Writing on Syria, and his recent experience as UN and Arab League envoy, he said the country's sectarian rifts were "as deep and bitter as those of Lebanon and on a scale that threatens a clash of sectarian animosities that could dwarf even those that shook Iraq after 2003" and estimated that thousands would die as a result of the conflict.