The discovery was made when Border Force officers using sniffer dog Mitzy searched the Bulgarian registered lorry at the port of Dunkerque in northern France.
The truck, which was waiting to board a ferry to the UK, was carrying coffins bound for a funeral director in Hounslow, west London.
The stowaways, all Eritrean nationals, were handed over to the French border police and the vehicle was allowed to continue onto its destination.
Paul Morgan, Border Force Director for South East & Europe, said: ‘This was an unusual type of discovery, but in the past we have found people hiding in an array of freight, ranging from dog biscuits, bathtubs and now coffins. The incident shows exactly why we base Border Force staff in France - to stop would-be illegal immigrants before they can reach the UK.
‘Our strong presence in French ports and close collaboration with the French authorities helps protect the whole of the UK from people attempting to enter the country illegally.
‘As well as using sniffer dogs, officers also use heartbeat detectors, carbon dioxide probes and physical searches to find people hiding in vehicles.’
A leaflet, issued by the Border Force, in partnership with the Serious Organised Crime Agency (SOCA), gives information and advice to lorry drivers transporting freight to and from the UK on how to prevent illegal immigration and avoid becoming victims of crime.