Ex-UN boss Kofi Annan has told the BBC Kenya has until the end of August to set up a special tribunal to try the ringleaders of post-election clashes.
Mr Annan brokered a power-sharing deal last year to end the violence in which some 1,500 people were killed.
A commission of inquiry then said the court should begin hearings in March.
Mr Annan says if the new deadline is not met, he will hand over the sealed list he has of the key suspects to the International Criminal Court.
In February, MPs rejected the bill to establish the tribunal despite intense lobbying by President Mwai Kibaki and his former rival, Prime Minister Raila Odinga.
A further two-month extension granted by Mr Annan has also passed.
“I’m in discussions with the two leaders… and they told me they’re going to make a second attempt to get the tribunal established,” Mr Annan told the BBC’s Network Africa programme.
“I’ve also made it clear that if it is not established within a reasonable period, which I would say [is] up to the end of August, I will have no option than to hand the envelope over with the names to the ICC to take it over from there.”
Opponents of the bill said they had no faith in Kenya’s justice system.
Some politicians have been accused of trying to delay the hearings until the 2012 elections.
Mr Annan said he hoped this was not the case as it was important for the accused “to be tried in the community [where] they committed the crime”.
Failure to do so, he said, would be a “big blow to the fight against impunity”.
“I think Kenya would be much better off with that trial taking place in their midst,” he said, urging parliamentarians to pass the legislation.
“They are collectively and individually responsible and they should work with the speaker and their fellow parliamentarians to establish the court for the sake of justice – the victims deserve justice.”
The violence broke out after Mr Odinga’s supporters said he had been cheated of victory in the December 2007 presidential election.
This sparked long-standing ethnic rivalries over access to land and other economic resources and some 300,000 people were forced to flee their homes.
After two months of nationwide violence, Mr Annan brokered a deal for Mr Odinga to join a power-sharing government as prime minister.
But thousands of people still live in camps for those displaced – they say it is still to dangerous for them to return home.
SOURCED FROM BBC