The Paris transport hub had to be evacuated early on Monday morning after a World War II shell was found in a passenger's bag.
The evacuation, which happened at about 5.45am on Monday, was completed by 10am, but has led to longer than usual queues for Eurostar services.
It is the latest in a series of security alerts caused by passengers trying to take souvenir war artillery on to trains.
Eurostar issued a special warning ahead of the First World War commemorations in November 2018, but with just a month to go until memorial events for the D Day landings, there are fears that the problems could recur.
“As you're travelling during the commemoration period, please remember that you can't bring any real or replica bombs, shells (complete or partial) or weapons on board – even if you bought them from a gift shop,” Eurostar told passengers last year.
“If you bring them with you, they'll be confiscated at security and may result in the need to evacuate the station.”
Old World War I shells turned into flower pots have been popular souvenirs in Belgium and France ever since the end of the conflict, but passengers attempting to take them on board trains for Britain have sparked bomb scares in recent years.
Some of the alerts, which happen every few months, have also been caused by war memorabilia collectors bringing back disarmed ordnance unearthed by farmers at battle sites in northeast France.
Eurostar said even disarmed shells can set off X-ray alarms.