Other witnesses heard a "massive rumbling" moments before a 20-metre stretch of the new tunnel caved in on the main London to Midlands line.

There were no reports of injuries but passengers face days of chaos after thousands of tonnes of concrete and steel crashed down at 7.30pm. Police said that further collapses were highly likely.

Charles Ward, 41, who lives locally, said that a high-speed train had passed through the tunnel only minutes before. "I was looking out of my window and suddenly there was this huge rumbling sound. I looked out and I could see the whole middle section had collapsed.

"It just does not bear thinking about if a train had been in the tunnel at that moment. Hundreds would have died."

The new tunnel has been at the centre of controversy since John Prescott, the Deputy Prime Minister, overruled local planning bodies and granted permission for a £20 million Tesco supermarket and car park to be built on top of it. The tunnel was constructed over a railway cutting to support the new structure above the tracks in the first engineering project of its kind in this country.

Thames Valley Police said that clearing the tracks of rubble and repairing them would cause very severe problems for commuters. Chiltern Railways was working hard to make alternative arrangements. "We have got people on the site looking at exactly what happened and to try to make some sense of it," a spokesman said.

He added that the driver of a stationary train at Gerrards Cross quickly advised his control and trains were stopped. "One train with about 70 passengers on board was stranded for about 15 minutes but was taken back to the nearest station at Denham Golf Club. Alternative transport was arranged by Chiltern Trains, the train operator on that line."

The Health & Safety Executive will investigate the cause of the collapse, he said. A spokeswoman for Tesco said last night that an urgent investigation was already under way.

The proposed store has been bitterly opposed by residents in Gerrards Cross, one of the country's wealthiest towns and home to an estimated 500 millionaires. In a referendum of more than 8,000 locals, 93 per cent of voters opposed the original applications, fearing that it would harm local stores, bring traffic chaos and spoil the character of the town.

For the past few months construction has been taking place in five-hour shifts at night after the last trains to and from London have passed through.

A specially constructed conveyor belt has been ferrying in tonnes of earth and 400 concrete arches to support the new shop and car park on top of the railway cutting.