Khaled Ali, 45, who also ran in Egypt's 2012 presidential election, made the comment during a meeting of opposition parties on Sunday to condemn the ongoing review of last year agreement, which handed over the two uninhabited islands Tiran and Sanafir to the Arab kingdom.
"The president is a traitor and the prime minister is a traitor," said Ali in his address before the meeting, a video of which was posted on social media on Monday. "Whoever will be content to lower the Egyptian flag on Tiran and Sanafir and raise the Saudi flag in its place is a traitor," he added.
Ali's remarks could now bring legal consequences for him over publicly insulting the president, and might put him on a bumpy road of running for Egypt's 2018 presidential election, in which he is most likely to stand as a strong rival to Sisi.
Being a vocal critic of the controversial accord, Ali also launched an appeal in early April against a court ruling, which lifted a block on the transfer of the islands to Saudis.
President Sisi announced in April 2016 that the two islands fell within the territorial waters of Saudi Arabia as stipulated in a border accord signed between Cairo and Riyadh earlier that month.
On Sunday, the government advised the legislators, who are preparing to vote on the hot-button issue, that the accord only "ends (Egyptian) sovereignty on the islands" but keeps administrative control of Egypt over the pair.
The deal, however, triggered unprecedented mass demonstrations, with protesters denouncing the arrangement as unconstitutional. A number of lawyers, including Ali, also filed a lawsuit in the administrative court to block the deal.
Protesters have accused the Egyptian leader of surrendering Egyptian territory in return for Saudi money amid reports that Cairo was receiving 20 billion dollars in aid from Riyadh to relinquish sovereignty of the islands.
Riyadh and Cairo argue that the islands belong to Saudi Arabia and that the Arab kingdom asked Egypt in 1950 to protect them. However, lawyers and opponents say Egypt’s sovereignty over the islands dates back to a 1906 treaty, before Saudi Arabia was founded.
Tiran and Sanafir lie about four kilometers apart in the Red Sea. They are situated in the narrow entrance to the Gulf of Aqaba leading to Jordan and the occupied Palestinian territories.