The prosecutor at the International Criminal Court is asking for an adjournment in the trial of President Uhuru Kenyatta of Kenya.
Fatou Bensouda said the loss of two key witnesses meant the case currently "does not satisfy the high evidentiary standards required".
The trial was due to begin in February next year.
Mr Kenyatta denies charges of organising ethnic violence after the 2007 election in which some 1200 people died.
In a statement, Ms Bensouda said a key witness had recently indicated he was no longer willing to testify and another witness admitted giving false evidence.
She said she now needed time to complete efforts to obtain additional evidence.
The BBC reports the trial of Mr Kenyatta was due to begin on 12 November, but was delayed after an attack in September on the Westgate shopping centre in Nairobi that left at least 67 people dead.
African leaders had been lobbying for the case to be delayed until Mr Kenyatta is no longer in office.
The African Union has accused the ICC of unfairly targeting the continent's leaders.
The trial of deputy president William Ruto began in September this year, although it was also adjourned.