Archbishop Desmond Tutu has pulled out of an international summit because he doesn't want to share a platform with the "morally indefensible" Tony Blair, it emerged yesterday.

The retired archbishop, who received the Nobel Peace Prize in 1984 for his campaigning against apartheid, said that he had withdrawn from the event because he believed the former Prime Minister had supported the invasion of Iraq "on the basis of unproven allegations of the existence of weapons of mass destruction."

In a statement, Archbishop Tutu's office added: "The Discovery Invest Summit has leadership as its theme. Morality and leadership are indivisible. In this context, it would be inappropriate for the Archbishop to share a platform with Mr Blair."

A spokesman added that it was not a snap decision, saying that the Archbishop "thinks and prays and then acts". He added: "That's how he's always done things, including during the struggles."

Mr Blair and Archbishop Tutu, alongside the chess grandmaster Garry Kasparov, were due to appear at the leadership summit in Johannesburg later this week. The Muslim political party Al Jama-ah has already said that it will attempt to arrest Mr Blair when he arrives in Johannesburg for "crimes against humanity".

Mr Blair's office said he regretted the decision. In a statement, it said: "Tony Blair is sorry that the Archbishop has decided to pull out now from an event that has been fixed for months and where he and the Archbishop were never actually sharing a platform.

"As far as Iraq is concerned they have always disagreed about removing Saddam by force – such disagreement is part of a healthy democracy.

"As for the morality of that decision, we have recently had both the memorial of the Halabja massacre where thousands of people were murdered in one day by Saddam's use of chemical weapons, and that of the Iran-Iraq war where casualties numbered up to a million, including many killed by chemical weapons. So these decisions are never easy morally or politically".

Archbishop Tutu has long been a critic of Mr Blair's stance on Iraq – even before the invasion.

In 2003 the archbishop said Mr Blair's support for the Bush administration was "mind-boggling". "I have a great deal of time for your Prime Minister, but I'm shocked to see a powerful country use its power frequently, unilaterally," he said.

After the invasion he called on Mr Blair to apologise for an error of judgement on Iraq. "How wonderful if politicians could bring themselves to admit they are only fallible human creatures and not God and thus by definition can make mistakes," he said. "Unfortunately, they seem to think that such an admission is a sign of weakness. Weak and insecure people hardly ever say sorry.