A 31-year-old Briton was arrested by French police in Coquelles after he was caught travelling through the Channel Tunnel from Kent on foot.
The man, whose identity has not been released by authorities, was seen disappearing into the mouth of the 31-mile tunnel at Folkestone a week ago.
He is thought to have scaled four security fences and dodged 400 surveillance cameras before running in darkness through the tunnel.
The 31-year-old may have accessed the service tunnel, which is accessible at the UK terminal in southern England and lies between the two rail tunnels.
He may also have walked along a narrow walkway on either side of the tracks in one of the the rail tunnels, which are connected to the service tunnel every 375 metres.
If he walked through the rail tunnels, he would have been forced to bear incredibly high temperatures, altered air pressure and roaring noise from 100mph trains.
Eurotunnel had warned that anyone trying to walk along the lines would be killed, with risk from both high-speed trains and potentially fatal electrical currents.
It is estimated that the disruption to traffic caused by the incident cost Eurotunnel – the Channel Tunnel operator – some £45,000 in lost revenue.
The man was arrested at Coquelles on December 21 and was fast-tracked through the French court system before appearing in court at Boulogne-sur-Mer last Wednesday.
A 31-year-old Briton was arrested by French police in Coquelles after he was caught travelling through the Channel Tunnel from Kent on foot (file photo)
The 31-year-old may have accessed the service tunnel, which lies between the two rail tunnels and is used to evacuate passengers in the event of an incident. He may also have walked along the narrow walkways at either side of the tracks, and would have been forced to bear high temperatures, altered air pressure and roaring noise from trains travelling at 100mph
The tunnels reach their lowest point at 115m below sea level, about eight miles offshore
French police sources claimed that he refused to give his name or address but was soon identified as a 31-year-old man from the UK.
The man was remanded in custody in France prior to another hearing on February 1 due to a lack of further information, sources added.
MP Damian Collins told MailOnline: I’m glad that the trespasser was caught before a nasty accident could happen, although I understand not before causing considerable disruption and loss in revenue to Eurotunnel.
‘This is a very rare incident, and I’d like to thank the British and French authorities for dealing with it so swiftly. It could have been a lot worse, I hope the already considerable security checks will be stepped up on both sides to prevent a reoccurrence.’
The incident has raised serious questions about border policing following Britain’s departure from the EU on January 1.
Speaking to MailOnline, Alp Mehmet, Chairman of Migration Watch UK, said: ‘It seems our borders remain as porous as ever. This latest breach of security shows how desperate the need is for investment in immigration control.
‘What we would all like to know is, when, exactly, are we going to take back control?’
It is only the second known incident of its kind, and happened despite massive security measures inside the undersea facility.
The Channel Tunnel is composed of three tunnels each 31 miles long bored at an average 40m below the sea bed, linking Folkestone to Coquelles in Calais.
It is only the second known incident of its kind. In 2015, Abdul Haroun (pictured) walked the length of the Channel Tunnel and was eventually granted asylum in the UK
Shuttles, Eurostar and freight trains travel at up to 100mph along the Channel Tunnel line and pose an immense danger to anyone walking in the Tunnel.
A spokesman for Eurotunnel said: ‘A person was detected inside the Channel Tunnel and taken into custody by the French Authorities.
‘As there is now a criminal procedure in place we are unable to comment further at this time.’
A Sudanese migrant who walked the length of the Channel Tunnel from France in 2015 was eventually granted asylum in the UK a year later.
Abdul Haroun was initially charged with obstructing a railway under 19th-century legislation before also being held in custody.
But the 40-year-old was instead granted asylum, leading to Eurotunnel saying in a statement: ‘He not only caused significant disruption to Eurotunnel and to the many freight and passenger customers travelling at the time, he also put his own life and that of others at risk.’