The passenger's eagerness to bring home some Second World War souvenirs led to the ferry terminal at Ouistreham, in Normandy being evacuated, the bomb squad called in a Brittany Ferries boat diverted, according to BFM TV.
The chaos began when French Customs guards stopped the Brit, aged around 40 and described as a "collector", when he tried to board a ferry with the shells hidden in his suitcase.
Police arrested the man, and the ferry terminal was immediately evacuated in order to bring in bomb disposal experts.
The man was kept in custody of the Caen police to “determine the origin of the shells”, police press officer Sylvain Briand told BFMTV.
A Brittany Ferries boat which the man had planned to take had to be diverted, but the company said it had not caused passengers any significant delay.
It's not the first time British collectors of World War Two memorabilia have caused transport chaos trying to get their artefacts home.
The Local previously reported how Gare du Nord rail station in Paris was partly evacuated after a passenger was caught trying to sneak a shell onto the Eurostar.
Archeological associations have also raised the alarm in the past about British collectors "pillaging" France of its historical treasures, notably remnants from the two world wars.
"In Normandy and part of northern France a lot of English will come over with metal detectors and scour the battle fields from the First and Second World Wars," Jean-David Desforges, head of the French association Stop the Pillage of Archeological and Historical Heritage (Happah), told The Local.
"It is the same along the border with Belgium, Luxembourg, and Germany where these people will cross into France to search war battlefields and take what they found back to their country."