Eurostar says all its passengers must wear face masks as a safeguard against Covid-19, effective from Monday.
In a tweet in French and in English, the international high-speed train service says the policy will apply to both its trains and its stations, “in line with guidelines announced by the French and Belgian governments”.
It warned passengers without a mask could be refused travel.
It says any mask is “suitable” if it effectively covers the nose and mouth.
The company has dramatically reduced its services connecting Brussels with Paris and London.
There are currently only four trains a day.