Ferry operator P&O has started an investigation after a car carrying four people fell off a ramp on one of its Hull-based ships.
The vessel had just completed its first overnight crossing of the year
The German family were in the car when it fell 6ft (1.8m) off an internal ramp as The Pride of York was unloading at Zeebrugge in Belgium on 2 January.
The two adults and two young children escaped injury but were taken to hospital for checks, P&O said.
The car was badly damaged after falling to the deck below.
P&O arranged alternative transport to take the family back to their home in Germany.
A spokesman for the company said: "This clearly should not have happened so we have begun a thorough inquiry to establish what went wrong."
He said engineers had inspected the vessel and would report to health and safety officials in the UK and Belgium.
"Thankfully the family were not injured but the car was badly damaged," the spokesman said.
"I have not come across any similar incident in my five years at P&O."