An urgent safety warning has been issued after lorries fell on their side during a ferry crossing in rough seas.
Nine lorries fell and cars were crushed when European Causeway was sailing from Larne in Northern Ireland to Cairnryan in Scotland in December last year.
Marine Accident Investigation Branch conducted an inquiry which found at least six drivers had stayed in their cabs despite being instructed to leave the roll-on/roll-off (ro-ro) deck after parking.
One of the drivers was trapped and needed to be rescued by the emergency services in Scotland - no one was hurt.
International regulations state that passengers are not allowed to remain on ro-ro decks as they are in danger of asphyxiation by fire or as a result of systems used to tackle fires.
The MAIB's investigation found that the problem of lorry drivers remaining in their vehicles on ferries is "not unique to this route or to P&O".
An urgent safety warning states that ro-ro decks are a "hazardous and potentially life-threatening environment" and should only be occupied by professional seafarers.
Drivers who remain on the deck may delay an emergency response to a fire, which could result in "catastrophic consequences to the whole vessel, its passengers and the environment", the MAIB added.
It recommended that the Road Haulage Association should alert its members about the European Causeway incident and encourage them to take "robust action" over the issue.