A high number of coaches returning children from half term ski trips were among those understood to have faced weekend delays due to striking Border Force staff.
Hold ups of more than six hours were reported at border checkpoints in Calais.
Border Force staff members of the Public and Commercial Services (PCS) union started a four-day strike in Dover, Calais, Coquelles and Dunkirk on Friday.
The PCS said Border Force members had been on a picket line at the Euro Tunnel service entrance in Folkestone, with “solid support for the action across the board”.
The Home Office brought in “lots of staff” from around the country to cover, according to the union, which is taking targeted action in a dispute over pay, pensions and job security.
P&O Ferries urged returning travellers to the UK to “plan for a wait by bringing snacks, drinks, entertainment etc” due to queues at border controls at Calais on Saturday.
It said that coaches and minibuses were the worst affected.
DFDS said it had been working with the Home Office and Border Force “to mitigate the impact of these strikes and to keep disruption to our customers’ travel plans to a minimum”.
A Home Office spokesperson told the BBC on Saturday: “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.
“Border Force and port operators are working hard to ensure all travellers have a safe and secure journey, however we have been clear those entering the UK should expect disruption during strike action.
“We continue to work closely with port operators at a local and national level to minimise delays.”
Port of Dover chief executive had praised Border Force contingency plans for strike days.