French officials say nineteen workers have suffered carbon monoxide poisoning while working overnight in the Channel Tunnel, with one of them in a serious condition.
The incident happened as some 60 workers were changing rail tracks on the line between Calais, France, and Folkestone.
A welder was taken ill in the early hours of Sunday and carbon monoxide poisoning was later diagnosed.
Tunnel traffic was unaffected.
Eighteen other workers have also been taken ill.
An investigation has been launched into the cause of the incident.