The intruder, who evaded security fences and cameras and dodged 100mph trains, was finally intercepted on Tuesday morning as he approached the French entrance to the tunnel near Calais. It is believed he entered the Cheriton tunnel mouth, near Folkestone, some time on Saturday.
His motive and state of mind remained a mystery last night. He told police he intended to join the French Foreign Legion.
Eurotunnel said it was "very worried" by the incident. "We are still asking ourselves how this man could have got into the tunnel and how he could not have been spotted for so long," a company spokeswoman said.
French police also said it was a "miracle" that the man had survived his 30-mile hike. Eurostar trains passing through the tunnel at 150km an hour (92mph) cause winds and a vacuum effect that should, in theory, suck up and kill any pedestrians near their path.
French and British police believe the man - who has been returned to Britain for questioning - may have walked through one of the two, main tunnel shafts during maintenance periods at night and hidden in the smaller, rescue tunnel and side passages at other times.
"At night parts of the tunnel are closed for maintenance and the trains that do run, mainly freight, move more slowly," a Eurotunnel official said.
French police said that the man did not seem crazy. He was not an adventurer or someone who had set out to walk from England to France for a dare. His motives remained a mystery.
Eurotunnel is urgently reviewing its security arrangements, especially its network of closed-circuit cameras. "If we had know that there was a pedestrian in the tunnel, we would have stopped the trains," the official said.Reuse content