Organised gangs are thought to be behind the theft of copper cable from the construction site of the Channel Tunnel Rail Link (CTRL) in Kent.
The rail link operator said anyone cutting live cables could be killed
Cable worth tens of thousands of pounds has been stolen by gangs breaking in to the site at Ebbsfleet.
"It is of relatively high value and is being disposed of outside our area through scrap metal dealers," said Sgt Rod Chapman of Kent Police.
British Transport Police said the thefts were part of a national trend.
The second stage of the link from north Kent to St Pancras is due to open next year.
The copper cable, which is used to power trains, stations and signals, is now being tested and CTRL operator London and Continental Railways said thieves could be killed.
"We do testing day and night and the copper on site could well be live," said spokesman Ben Ruse.
"If you cut through a copper cable carrying 25,000 volts that will be the last bit of copper you cut through."
Hundreds of contractors enter the site every day and it is thought some thieves may be getting in using forged passes.
Others cut through the wire fence surrounding the site.
Thefts have increased as the price of copper has risen. Two years ago the official price of scrap copper was £1,700 per tonne - now it is £4,500.
London and Continental said site security had been increased and the cable was now being stored at a secret location before installation.