South Africa’s year-old role as chief mediator in the Zimbabwe crisis was thrown into doubt last night after the Zimbabwean opposition leader, Morgan Tsvangirai, called on President Mbeki to step down.

In a sign of the increasing divisions in the region over South Africa’s policy of “quiet diplomacy” towards Zimbabwe’s autocratic president, Robert Mugabe, Mr Tsvangirai said he had called on the Southern African Development Community, the regional grouping, to lead a new initiative led by the Zambian government.