No-one was injured in the accident, although a train heading towards the tunnel when it collapsed had to perform an emergency stop.

Early reports suspected the cause was the backfilling operation; the Health and Safety Executive are yet as of December 2020 to release their findings owing to legal issues.

In July 1998, however, Deputy Prime Minister John Prescott overturned the council's ruling on the basis of the recommendations of an independent planning inspector.

[9][10] Babcock were still subject to the parent company guarantee that Peterhouse Group had entered into when the contract to build the tunnel was let.

"[6] At the time of collapse, construction work on the tunnel segments had finished, and the backfilling operation was taking place.

The train had passed through Denham Golf Club station, the last stop before Gerrards Cross, before the driver was warned by radio.

[citation needed] Additionally, a London-bound train was standing at the eastbound platform at Gerrards Cross waiting to depart.

One witness told a news channel that he was parking his car when he heard "what sounded like a clap of thunder - I thought it was an earthquake.

"[17] Reg Whittome, chairman of the Marylebone Travellers' Association and the Chiltern Railway Passenger Board, said after the incident that, had a train been going through the tunnel when it collapsed, there would have been an "almighty tragedy".

[12] Extensive design checks were carried out to ensure that the sections of tunnel lining that had not collapsed were safe.

"[17] At the time of the accident, James Ford added that no further work would be carried out on the tunnel unless it had been certified as safe.

Tesco may, however, choose to continue work on parts of the supermarket project that are not directly connected to the tunnel.

The tunnel segments allowed for substantial vertical settlement during construction, and it was vital that the material was built up evenly on both sides and over the crown of the arch.

"The collapse was probably due to too much load on the crown of the arch and not enough fill on the sides," the head of a specialist consulting firm said.

At the section that is still standing the central hinge has bent down indicating an imbalance between the vertical and horizontal loads and a failure by outward spreading of the arch.

"[7] He added that heavy rainfall, which had hit Gerrards Cross several days before the collapse, may have made the problem worse.

The local Conservative MP, Dominic Grieve, said: "If this particular disaster encourages Tesco to not go ahead with the scheme, everyone in Gerrards Cross will be absolutely delighted.

"[6] Peter Hardy, the leader of South Bucks district council, added after the landslide that there would be "further years of disruption ...

One protester said they were "fighting to get Tesco to tell us exactly why the tunnel collapsed, and to stop them from succeeding in winning further planning permission to continue work on the store."

[22] The store manager, Lizzie Field, said: "We are really pleased to support our local community at a time when so many people are looking for jobs, and there is so much competition between applicants.