The Belgian rail network will be closed on Thursday, as public sector workers take part in a general strike in protest against pension reforms.
Eurostar has said its services to and from Brussels will be disrupted for 24 hours from 22.00 on December 21.
Over the period, the rail operater will run trains as far as Lille Europe station, with a limited replacement coach service to take passengers to and from Brussels Midi station.
Passengers are being warned, however, that demand “is likely to be extremely high for these coaches and we expect both stations to become busy as a result”.
Eurostar is recommending that passengers booked to travel during the strike change their tickets to either before or after December 22.
The rail operator has said it will allow exchanges “free of charge regardless of ticket conditions but subject to availability on alternative services”.
Eurostar’s trains to and from Paris will not be affected.