Sixty people, including workers from a company commissioned by Eurotunnel and Eurotunnel employees, were in the process of changing rails on the line between the towns of Calais in France and Folkestone in Britain when a welder took ill overnight, said officials from France's Pas-de-Calais department.
"Carbon monoxide poisoning was diagnosed," an official said, adding the worker, who was "quite seriously affected", was taken to a hospital in Calais then Lille in northern France.
Eighteen other workers were also taken ill and sent to hospitals in Calais, Boulogne-sur-Mer and Dunkerque, "the time needed to remove the carbon monoxide from their system", an official said, adding their condition was not a cause for concern.
An investigation has been launched into the cause of the incident.