An Egyptian court has ruled as unconstitutional a clause in the election law that draws voting districts.
The verdict is almost certain to delay parliamentary elections scheduled to start later this month.
President Abdel-Fattah el-Sissi said in a statement that he has ordered the government to adopt the "necessary legislative amendments" within a month to comply with the Supreme Constitutional Court's ruling.
Another court is likely to rule later this month on whether the election should be delayed, but a date for the new vote would be decided by the Supreme Electoral Commission, which went into an emergency meetingtoday following the verdict.
Egypt has not had an elected legislature since 2012, when the Supreme Constitutional Court ruled that the lower chamber was not constitutionally elected. Islamist supporters of then-President Mohammed Morsi besieged the court ahead of a hearing in which it was expected to issue a similar ruling against the upper chamber, preventing the judges from reaching their chambers.
The forthcoming election is the third and final step in a road map announced by Mr el-Sissi when he ousted Mr Morsi in July 2013 amid massive protests against the Islamist leader's year long rule.
The first two steps were the adoption of a new constitution by referendum in 2014 and a presidential election that was comfortably won later that year by Mr el-Sissi, who has ruled by decree since coming to office in June.
Mr Morsi's Muslim Brotherhood swept the first free parliamentary elections after the 2011 uprising that toppled long time autocrat Hosni Mubarak. The group is now officially considered a terrorist organisation, and thousands of its members, including most of its top leaders, are in jail.