Three candidates including two former presidents have withdrawn from the Burundi's upcoming presidential race predicting the contest will not be free and fair.

The letter announcing their withdrawal from Tuesday's election is the latest blow to the country's political system which has been rocked by months of violence after the incumbent President Pierre Nkurunziza announced his bid for a third term.

Former presidents Domitien Ndayizeye, Sylvestre Ntibantunganya and former Speaker of Parliament Jean Minani said in the letter to the electoral commission that the political and security environment could not guarantee free and fair elections.

Prosper Ntahorwamiye, the commission spokesman, said Saturday he had received the letter but the names of the candidates will remain the ballot papers which had already been printed and distributed.