The decision reflects concerns that the rushed process leading to Saturday's vote has undermined the legitimacy of the disputed charter backed by President Mohammed Morsi and his Islamist allies.
The speedy referendum has left the country deeply polarized.
The Carter Center said Thursday that it would not be able to conduct "a comprehensive assessment of all aspects of the referendum process."
Most Egyptian judges also are boycotting the referendum, saying they will refrain from overseeing the process as stipulated by law. In response, Morsi issued a decree breaking the vote into two rounds - one on Saturday and another on Dec. 22.
(Copyright 2012 by The Associated Press. All Rights Reserved.)