Kofi Annan has urged Kenyans not to vote for politicians facing trial at the International Criminal Court (ICC) in next year’s election.
Mr Annan, former UN secretary general and now African Union envoy overseeing the election, said Kenya’s external relations could be damaged.
Candidates Uhuru Kenyatta and William Ruto are due to stand trial after the vote in March 2013.
Correspondents say the intervention is likely to infuriate them.
Although he did not name Deputy Prime Minister Kenyatta and former minister Ruto, Mr Annan told the BBC that any Kenyan leader must be able to travel to meet other heads of state and be trusted by the international community.
“When you elect a leader who cannot do that, who will not be free or will not be easily received, it is not in the interests of the country and I’m sure the population will understand that,” he said.
Both Mr Kenyatta and Mr Ruto have been indicted by the ICC over deadly post-election violence in 2007.