Prime Minister Rishi Sunak told reporters on Friday that many armed forces personnel were likely to miss Christmas "to help us deal with the disruption from strikes, whether that's manning border posts or driving ambulances."
Asked if the military would definitely be deployed, a defence ministry source said they would be "held at readiness".
Britain faces an unprecedented number of strikes in the run up to and during the Christmas holiday period, including nurses, ambulance staff and rail workers, as labour unions demand higher pay for their members to keep up with soaring inflation.
The PCS trade union, which represents thousands of Britain's Border Force workers at several major airports, said staff would strike for eight days this month in a dispute over pay.
Interior minister Suella Braverman warned on Thursday that Border Force strikes may cause delays, adding that people should think carefully about their plans to fly abroad.
(Reporting by Farouq Suleiman, Editing by Kylie MacLellan)