A spokesman for Eurostar told the Daily Mail that it was "relatively common" for passengers to attempt to bring such items on board.
"The delays yesterday were caused by some memorabilia which had been brought back from the war sites in Northern France.
"On this occasion someone had done so without declaring it to Customs, so there was an evacuation whilst this was checked," he said.
"The object was found when all bags were scanned, as part of the regular check-in procedure."
David Lean, a passenger caught up in the delays, told the Daily Mail his journey home was delayed by more than two hours and he arrived in London three hours late.
"We were among the lucky ones as I believe a lot of others were stranded in Paris overnight," he said.
Eurostar cancelled the last two trains of the night and a special train service was brought in to bring stranded passengers to London.
24-year-old Jak May told the Evening Standard the incident cost him more than £200 [$427], as he had to book his party into a hotel for the night.
"The communication has been appalling. We had to beg Eurostar officials to talk to us in English. It was like they were scared to give out any information at all," he said.
"I personally think whoever is trying to bring the shell back is beyond disrespectful especially with how tense Paris is at the moment. As a group we saw the heightened police presence and were genuinely worried."
While military relics are easily purchased in Europe - shells are regular found in farmers' fields - they are banned on Eurostar trains.
Eurostar's website states, "If you've been to visit one of the commemorative First World War sites, you've probably noticed that you can buy certain military relics from this era as souvenirs.
'In accordance with Article 215 of the French Customs Code, you'll be at risk of arrest, prosecution and a substantial fine if you attempt to travel with any historical weapons, munitions or explosive devices, either within your luggage or carried on you.
"Such restrictions are also applied to carrying military shells and cartridges (even if they appear to be empty).
"These items are a potential danger to yourself and all others in the vicinity where they're carried. They have also been the cause of frequent station evacuations and significant travel delays when detected.
"We will also disregard any certification from sellers of such items claiming that they have been de-activated and are therefore harmless. The police will deal with each detected item in the same way."
- nzherald.co.nz
