Archbishop Desmond Tutu has withdrawn from an event later this week in Johannesburg because he feels he cannot share a platform with Tony Blair.

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retired archbishop, who received the Nobel Peace Prize in 1984 for his campaigning against apartheid, said that he had withdrawn from the event because he found the former prime minister's support for the invasion of Iraq to be "morally indefensible".

A statement from his office explained:

Ultimately, the Archbishop is of the view that Mr Blair's decision to support the United States' military invasion of Iraq, on the basis of unproven allegations of the existence in Iraq of weapons of mass destruction, was morally indefensible. The Discovery Invest Leadership Summit has leadership as its theme. Morality and leadership are indivisible. In this context, it would be inappropriate and untenable for the Archbishop to share a platform with Mr Blair.

A spokesman for Archbishop Tutu told me that this should not be viewed as a snap decision, saying that Tutu is "a very prayerful man" who will have "spent hours on his knees considering this decision". "He thinks and prays and then acts," he said. "That's how he's always done things, including during the struggles."

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

A spokesman for Tony Blair said: "Obviously 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."