MANILA (Reuters) - An unexplained power outage at Manila airport's Terminal 3 on Monday led to about 40 domestic Cebu Pacific flights being cancelled, the airport said.

"Flight delays are expected due to the outage", the Ninoy Aquino International Airport said in a statement posted on Facebook, without disclosing the cause of the power failure.

Standby power systems were supplying electricity to critical facilities enabling airline and immigration computers to function partially and enable processing of both inbound and outbound passengers, it said.

The airport operator and supplier Manila Electric Company were looking into the cause of the power failure, which comes during a peak travel season, with many Filipinos about to return home from three-day weekend trips.

In a statement, Cebu Pacific offered its passengers departing to and from Terminal 3 an option to rebook or to request refunds to be put in a travel fund for future use.

President Ferdinand Marcos Jr has instructed Transportation Secretary Jaime Bautista to restore normal power operations at the airport's Terminal 3 as soon as possible while providing assistance to affected passengers, his office said.

(Reporting by Akanksha Khushi in Bengaluru and Enrico Dela Cruz in Manila; Editing by Christian Schmollinger and Kenneth Maxwell)