Eurostar passengers will be required to cover their faces from Monday 4 May or risk being refused travel.
The rail company said the rule for travellers to wear face coverings is in line with guidelines from the French and Belgian governments.
Any type of face covering is allowed "as long as it effectively covers your nose and mouth", a statement said.
It comes after Prime Minister Boris Johnson said face coverings "will be useful" as the UK eases lockdown.
Mr Johnson's comments followed a Scottish government recommendation for people to cover their faces when in shops and on public transport.
In its statement, Eurostar said fines may be imposed in France and Belgium for anyone without a face covering.
The company is operating a significantly reduced service, in line with increased border controls and a lower demand for travel triggered by the coronavirus pandemic.
Just four services are running each day between London and Paris, and London and Brussels, according to timetables published on the company's website.
Eurostar is a UK-based company but its main shareholder is the French state railway, SNCF. The French government has said face coverings will be mandatory on public transport when it begins to ease lockdown restrictions on 11 May.
The World Health Organization's (WHO) current advice says two groups of people should wear protective masks, those who are:
- sick and showing symptoms
- caring for people suspected to have coronavirus
There are concerns that wearing a mask may offer a false sense of security and lead people to be less careful regarding social distancing and other hygiene measures, such as washing hands.
The WHO said countries must weigh the risks and benefits when it comes to advising the whole populations about wearing face coverings.