Archbishop Desmond Tutu has pulled out of a leadership event in South Africa - because he refuses to share a platform with former British prime minister Tony Blair.
Nobel Peace Prize winner Archbishop Tutu said Blair's support for the invasion of Iraq was "morally indefensible."
The two were due to appear at the Discovery Invest Leadership Summit in Johannesburg, alongside chess grandmaster and Russian activist Garry Kasparov.
A statement from his office said: "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.
"The Archbishop greatly regrets inconveniencing and disappointing the organisers and participants of the Discovery Invest Leadership Summit."
The Archbishop has already been removed from the conference website.
A spokesman for Blair's office said in a statement: "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.
“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".
Hylton Kallner, Chief Marketing Officer, Discovery Holdings, told The Huffington Post UK: "The Discovery Invest Leadership Summit is a platform for local and international thought leaders to express diverse views and stimulate discourse.
"Archbishop Desmond Tutu's contribution would have been invaluable and it is deeply regrettable that he will not be attending the Summit. At no point had Archbishop Tutu indicated his discomfort at speaking at the same conference as Prime Minister Blair and therefore his withdrawal was unexpected.
"Archbishop Tutu is an esteemed global icon and his presence at the Summit will be missed."
The visit to Johannesburg had already been marred with controversy for the former prime minister.
Muslim political party Al Jama-ah has threatened to arrest Blair when he lands in the country, accusing him of will attempt "crimes against humanity".
The party is planning to hold a protest outside the Sandton Convention Centre on Thursda when Blair is due to speak.
Al Jama-ah leader Ganief Hendricks said: "The demonstration is being held to support a warrant of arrest to charge him for crimes against humanity relating to the invasion of Iraq which led to the killings of millions of Iraqis.
"It is hoped that one or more demonstrators will be able to make a citizen's arrest on the day and put Tony Blair in jail and extradite him to the Hague for trial."
South African media reported that a website, arrestblair.org, was offering financial rewards if anyone manages to make a citizen's arrest.
It is not the first time the archbishop has criticised Blair's support for the war in Iraq.
In 2003, he called the decision to go to war "mind-boggling" and said he was shocked and saddened America was being "aided and abetted" by the UK government.
He said: "When does compassion, when does morality, when does caring come in?
"I just hope that one day that people will realise that peace is a far better path to follow.
"The United States says you do this to the world, if you don't do it we will do it - that's sad."
Five years after Blair resigned as prime minister, he has been dogged by activists hell bent on making him pay for his errors over Iraq.
In 2010, Kate O' Sullivan tried to "arrest" Blair at a book signing in Ireland, and Tom Grundy, a Briton living in Hong Kong, walked up to Blair during an address at a local university and tried to make a citizens arrest in June this year.
He told The Huffington Post UK: "I want him to fear that wherever he goes and I hope it may stick one day."
Blair's testimony to the Leveson Inquiry in May 2012 was interrupted by an activist David Lawley-Wakelin who shouted that the former PM should be arrested for war crimes.