The Public and Commercial Services (PCS) union – which represents 15,000 Border Force, passport, and visa staff – is expected to announce its December strike dates on Wednesday (7 December).
The government has reportedly put 600 military personnel and a number of civil servants on standby to be deployed to airports to help alleviate pressures.
But according to industry bosses, the reinforcements "will not be enough" to avoid a number of flight cancellations over the festive period.
An airport chief told The Telegraph: "Border Force haven’t given us any indication of what resources might be available or how many of their staff might be called out on strike.
