A tunnel has fallen on to a rail line after 20m of a supermarket car park collapsed in Buckinghamshire.
Two trains were travelling in opposite directions when the Tesco tunnel at Gerrards Cross collapsed. The train drivers reported the damage.
Network Rail said there were no reports of injuries. Minor roads have been closed in the area and services on the Chiltern Line have been suspended.
It is believed that trains may not be able to use the line for three days.
Network Rail has sent engineers to the site where Tesco are building a store.
Eyewitness Brett Jackson told BBC News 24: "There is basically total mayhem at the moment.
"The police are trying to get everyone away. It still looks very unsafe. It looks like the rest could collapse any second.
"Apparently one of the trains missed it by minutes."
He said the tunnel collapse happened at about 1915 BST on Thursday. It is thought that tons of rubble has fallen onto the track from the Tesco construction site.
Urgent investigation
A Tesco spokeswoman said: "We can confirm that a section of rail tunnel at our new Gerrards Cross site has collapsed.
"Thankfully we understand that no one has been hurt but rail services are obviously disrupted, for which we apologise.
"We have launched an urgent investigation with our engineer contractors and will work with the authorities to understand what happened and get rail services back to normal as soon as possible."
The store was in the "very early" stages of construction and was being built above the line.
The Health and Safety Executive will be involved in an investigation and British Transport Police said trains would be disrupted for up to three days.
The line carries services between London and Midlands.
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Is this a new tunnel built to support the store overhead or an old one they just overloaded? Some f***ing idiot has made a SERIOUS planning error...bloody hell!
