Peace icon Desmond Tutu on Tuesday said he would boycott an event in protest at sharing a stage with former British leader Tony Blair over his "morally indefensible" support of the US-led Iraqi invasion.

The Nobel Peace Prize laureate decided to withdraw as a speaker at a one-day leadership summit after "wrestling with his conscience and taking counsel", his office told organisers.

"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," his office wrote to organisers.

The theme of the one-day summit on Thursday was leadership which could not be separated from morality, the letter added.

"In this context, it would be inappropriate and untenable for the archbishop to share a platform with Mr Blair."

Blair expressed his regret at Tutu's withdrawal from the one day event where the pair were part of the individual speaker line-up alongside chess champion Garry Kasparov and South Africa's Finance Minister Pravin Gordhan.

"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," his office said in a statement.

The former prime minister's office acknowledged the pair's different approaches to Saddam Hussein's regime.

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

Regarding the "morality" of his decision, the statement referred to recent memorials for victims of chemical weapons in the region and said: "So these decisions are never easy morally or politically".

The US-led coalition overthrew the Iraqi dictator in 2003 but also unleashed internecine violence that killed tens of thousands of people.

Well known for his outspokenness, Tutu's withdrawal comes amid plans by a local Muslim political party Al Jama-ah to protest at Blair's participation, linked to the decision to send troops into Iraq in 2003.

"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," Sapa news agency quoted the party's president Ganief Hendricks as saying.

Blair is the final guest speaker at the Discovery Invest Leadership Summit whose organisers expressed regret at Tutu's "unexpected" withdrawal, saying his contribution would have been invaluable.

"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," said Hylton Kallner, Discovery's chief marketing officer.

"Archbishop Tutu is an esteemed global icon and his presence at the Summit will be missed."