Those with a valid visa to enter the UK and a Ukrainian passport can request a ticket from Eurostar staff at Paris Nord, Brussels-Midi, Lille Europe or Amsterdam Central stations.
The decision follows news on Tuesday (1 March) that the UK could take in more than 200,000 Ukrainian refugees after the government relaxed visa rules to help those fleeing the war.
The scheme allowing close relatives of Ukrainians settled in the UK will be widened to include adult parents, grandparents, children over 18 and siblings and UK companies will also be able to sponsor a Ukrainian entering the country.
