Eurotunnel has halted all Channel Tunnel re-railing operations after more than 30 workers were taken to hospital with suspected carbon monoxide poisoning on two successive nights.
The workers - all of whom are French - were carrying out routine maintenance work on a section of the tunnel 10km from the French portal at Sangatte on the nights of 25 and 26 January.
Operator Eurotunnel has cancelled all welding and grinding work until it has the results of an internal investigation into the causes of the problem.
“Other work will go on in the tunnel but not using that equipment [for re-railing],” a Eurotunnel spokesman told NCE. “Re-railing will not carry on until we know what happened.”
The first signs of a problem came on 25 January when a worker became ill and was taken from the site by in-tunnel paramedics.
The remaining 66 workers on the project were checked over and 19 staff were taken to hospital for observations. All 19 - including the person originally treated by the paramedics - were later released.
Checks in the tunnel showed that all detectors were registering levels of carbon monoxide below the 200 parts per million threshold that is considered the safe limit.
However, when the work continued on Sunday night another worker became ill and was taken to hospital, along with 12 others, before all 13 were released from hospital.
Again the only cause for concern was the symptoms displayed, as there was no evidence of excessive carbon monoxide in the tunnel.
Re-railing work in the Channel Tunnel is almost continuous as track is heavily used. Up to 300 trains travel the route each day at speeds up to 250km/h, often with heavy loads. More than 20,000 detectors are installed in the tunnel to ensure safe conditions for workers and the public.
The French Labour Inspectorate is investigating the incidents, and the UK’s Office of Rail Regulation is being kept informed of developments.
The incident took place not far from the site of a fire in September 2008 that shut the route for a month.
Then, an open-sided truck wagon caught fire in the tunnel, triggering a 16-hour blaze, 11km from the French portal.