Ruling on transfer of Tiran and Sanafir comes a week after parliament backed plans to hand them back to Saudi Arabia.
Egypt’s top constitutional court has ruled to halt all verdicts on an islands transfer deal until it makes a decision on the constitutionality of the agreement, state media said.
Wednesday’s ruling came a week after Egypt’s parliament backed plans to hand over two uninhabited Red Sea islands to Saudi Arabia under an accord that has attracted widespread public criticism.
President Abdel Fattah el-Sisi must still ratify the agreement.
Parliamentary leaders and government lawyers say the House of Representatives is the only entity allowed to rule on matters of sovereignty, but it has been opposed by one court.
On Tuesday, an Egyptian court ruled that previous judicial decisions in favour of transferring the two islands were invalid, according to a judicial official and a lawyer.
The government says the two islands, Tiran and Sanafir, belong to Saudi Arabia but had been leased to Egypt in the 1950s.
On Tuesday, an administrative court ruled that all judicial decisions taken to date by the urgent matters court would be considered invalid.
"The ruling signifies that the land is Egyptian," said Khaled Ali, a lawyer who argued in the administrative court that the islands belonged to Egypt.
The verdict would affirm that any attempt to transfer the islands would be considered unconstitutional "even if the president ratified the agreement", Ali said.
The deal sparked rare street protests after it was signed last year, with President Sisi accused of having sold the islands to Saudi Arabia.
"Now, the ball is in the president’s court," Ali said.