The top of the structure, which ran alongside rail tracks at Kirkgate Station, partially gave way and fell into the car park of guttering and facia suppliers Trims Ltd on Calder Vale Road.

Mike Redshaw, the company's managing director, said he had reported concerns about the safety of the wall, which was several metres high, to surveyors examining the structure only weeks earlier.

The collapse also blocked a road providing customer access to the company's scrapyard. And a skip was shunted across the yard by falling rubble.

Mr Redshaw, 51, said it was lucky a scrapyard worker, who had gone inside only moments before, had not been badly injured.

He said: "It's just fortunate nobody was hurt."

The car crushed in the collapse belonged to wages employee Christine Lindley, 53.

Only days earlier Mrs Lindley had paid for her three-year-old Vauxhall Corsa's MOT and tax disc.

She said: "I was in the office and we heard this noise, it was like an earthquake.

"We saw the top of the wall had come down, then someone shouted that my car had been damaged.

"I felt sick, it's my baby, it's only a little Corsa, but I love it."

A spokeswoman for Network Rail, the company responsible for the wall, said the remainder of the structure was safe and work had immediately started to repair the damage.

She said: "As with all incidents of this type there is a large amount of investigation to be carried out, especially into speculation about tip-offs.

"This will of course form an important part of the investigation and the pieces of the jigsaw are only now being pulled together."