Egyptians will spend Tuesday and Wednesday voting for a constitution to replace the one adopted during the tenure of ousted President Mohamed Morsi.
"Protests and disruption to travel around the country are likely," the British advisory read. "You are advised to take particular care in the vicinity of polling stations and government buildings, and to avoid all demonstrations and large gatherings."
The interim government drafted a new constitution to replace Morsi's, which was seen as favoring the ideologies of his Freedom and Justice Party, the Muslim Brotherhood's political arm.
Morsi was removed from power by the military last year.
U.S. Defense Secretary Chuck Hagel spoke by phone with Egyptian Defense Minister Gen. Abdel Fattah al-Sisi ahead of the referendum.
"They discussed the importance of the constitutional referendum for Egypt's overall political transition, and Secretary Hagel stressed the importance of a transparent referendum in which all Egyptians have the opportunity to cast their vote freely," the U.S. Defense Department said in a statement Sunday.
Egyptian expatriates were able to cast their ballots last week. The Muslim Brotherhood issued a statement saying it was calling for a boycott of the referendum.