Eurotunnel officials are investigating into why 39 Channel Tunnel workers had to be treated for carbon monoxide poisoning on two consecutive nights over the weekend.

One of the workers was in a serious condition, French officials said on Sunday, but all those affected have now been released from hospital.

The first incident happened as some 60 workers were changing rail tracks on the line between Calais, France, and Folkestone, UK on Saturday night.

But there was additional concern after another incident happened on the following night (Sunday into Monday) when 66 workers were in the tunnel.

One person was taken ill, a further 13 were taken to hospital as a precaution while another 52 showing no symptoms were allowed to return home.

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 were also taken ill after it appeared the carbon monoxide detectors inside the tunnel failed to register levels were dangerous.

They were all sent to local French hospitals to give them "the time needed to remove the carbon monoxide from their system", an official said, adding their condition was not a cause for concern.

The 41 other workers went home.

An investigation has been launched into the cause of the incidents.

John Keefe, director of public affairs at Eurotunnel Group, confirmed all the staff were French and none were from Kent.

Mr Keefe said: "Detectors in the tunnel didn't show excess levels of carbon monoxide.

"A number of workers were taken into hospital for medical observation as a precaution and Eurotunnel is monitoring their condition carefully.

"Everyone is fine and all have been released from hospital.

"Our readings show that there was not an excess and we're not entirely sure what's happened. Once is worrying but twice is very concerning. There's a lot of work going on to sort that out.

"Friday, Saturday and Sunday are the main maintenance nights and carry less traffic. We relay the track more than any other railway. We've done this loads of times and never had any problems.

"This is the first time it's ever happened."

He added it was unclear how long the investigation would last but time was a secondary concern over finding out the exact cause.