Around 1,000 officials, members of the Public and Commercial Services Union, working at the four ports walked out on Friday 17 February.
PCS general secretary Mark Serwotka warned the action "would impact people returning from their holidays". P&O Ferries reportedly advised passengers arriving at the checkpoints to "come prepared" with refreshments.
According to the BBC, the Home Office said it was working to minimise wait times, but rejected claims delays were a result of the strike action.
A spokesperson said: "The queues at the Port of Calais today are not due to industrial action. Border Force operations there remain fluid with all booths open and no significant wait times."
