UK accident investigators have warned drivers to leave their vehicles while at sea following a dangerous incident on a P&O ro-pax last year.
The 20,600-gt European Causeway (built 2000) encountered heavy weather and seas en route from Northern Ireland to Scotland on 18 December.
The UK Marine Accident Investigation Branch (MAIB) said the roll caused nine of 40 freight vehicles to topple over.
The MAIB investigation found that at least six drivers had remained in their lorries during the crossing, despite instructions to leave.
Four were found in overturned lorries, with one trapped until he could be freed by the emergency services waiting in Cairnryan.
Nobody was hurt during the accident.
MAIB said its investigation revealed that drivers remaining in their vehicles on the ro-ro deck while ferries are on passage is not unique to this route, or to P&O.
"The ro-ro decks should be occupied by only trained professional seafarers who are required to undertake safety and security patrols," it said.
"Drivers who remain on the vehicle deck pose a danger to themselves, and can cause a delay to the emergency response, particularly in the event of a fire."
It added: "Any delay to the activation of fire suppression systems on the vehicle deck due to the need to undertake a muster and headcount of all persons onboard could have catastrophic consequences to the whole vessel, its passengers and the environment.
"Furthermore, drivers who have remained in their vehicle cabs could be in danger of asphyxiation by the fire, or as a result of the fire suppression systems that may be released by ships' staff."
