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.
The BBC's Gabriel Gatehouse in Nairobi says that many governments have said they simply will not deal directly with politicians who are under indictment at the ICC.
He says Mr Annan's comments are like to infuriate Mr Kenyatta and Mr Ruto, but it is not clear how much they will affect voters on polling day.
Although the Ghanaian former UN Secretary-General is a respected figure, many Kenyans still vote along community lines.
The two candidates in question - formerly bitter political rivals - have announced that they are forming an alliance for next year's election.
At a rally in western Kenya on Tuesday morning they said Mr Kenyatta would be the presidential candidate and Mr Ruto his running mate.
They were on opposite sides in the last election in 2007, but the ICC has indicted both men in relation to the violence that followed the poll.
Some 1,200 people were killed and 300,000 forced from their homes in the clashes that followed the disputed election.
Mr Kenyatta and Mr Ruto both deny any involvement in the violence.
At their joint rally on Tuesday in the Rift Valley, Mr Ruto said: "The ICC cases have formed the basis of debates but we are going to prove them wrong [and show] that Kenyans are united and want peace."