Nobel Peace Prize laureate Archbishop Desmond Tutu has withdrawn from a seminar in South Africa this week in protest against the presence of former British prime minister Tony Blair.
A spokesman said that Archbishop Tutu viewed Mr Blair's support for the war in Iraq as "morally indefensible".
Mr Blair strongly supported US President George W Bush as he launched a "war on terror", sending British troops to Afghanistan in 2001 and, more controversially, Iraq in 2003.
Archbishop Tutu won the Nobel Peace Prize in 1984 for speaking out against white-minority Apartheid rule and remains a prominent campaigner for peace and human rights.
"Morality and leadership are indivisible. In this context, it would be inappropriate and untenable for the archbishop to share a platform with Mr Blair," spokesman Roger Friedman said.
Mr Blair's office in London said he was "sorry" that Archbishop Tutu had decided to pull out of the Discovery Invest Leadership Summit in Johannesburg.
"As far as Iraq is concerned, they have always disagreed about removing (former Iraqi President) Saddam (Hussein) by force - such disagreement is part of a healthy democracy," a spokesman for Mr Blair said.
"These decisions are never easy morally or politically."