Tonnes of earth dropped on to the track at Gerrards Cross, Buckinghamshire, near to where a new Tesco store is being built.

Luckily, no trains were in the tunnel at the time on Thursday - though one was stationary in a nearby station.

However, commuters heading for London's Marylebone station will be affected by the incident.

Chiltern Railways said in a statement: "Safety inspectors are currently on site investigating what has happened.

"Passengers are being advised to travel via alternative routes and buses have also been ordered. We are currently putting arrangements in place to minimise disruption.

"We are currently making special arrangements to ensure passengers will be able to get into London via an alternative route on 1 July and we will provide further information as soon as it is available."

Network Rail said there were no reports of injuries after around 20 metres of tunnel collapsed.

Eyewitness Brett Jackson told BBC News 24: "I was parking my car when I heard what sounded like a clap of thunder - I thought it was an earthquake.

"I saw the tunnel falling on to the rail track."

He said the collapse happened at about 1915 BST on Thursday.

Urgent investigation

A spokeswoman for Network Rail said: "A train driver reported a partial collapse in the Tesco Tunnel.

"Two trains in the vicinity were immediately stopped. Expert engineers are now at the scene."

She said trains were being terminated at High Wycombe and West Ruislip and a bus service was being operated.

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.