Westminster Magistrates Court heard that Adrian Murray, 48, who lives in Ireland, was one of a small group of workers on the night shift spraying concrete on a three-metre diameter service tunnel between the Dorchester and 45 Park Lane.

Concrete was sent down steel pipes via a connector device to a flexible hose with a nozzle attached, enabling to operatives to spray the tunnel’s surface.

When the shift ended, they cleaned the pipes by using compressed air to shoot a foam ball and water down to the nozzle. The hose was secured and hung over a skip to catch the waste.

When the skip was full, the hose was moved to expel to the ground for one final clean through.

But as it ran down the pipe, the foam ball got jammed at the connector.

One worker rapped it with a spanner to free it but the build-up of the compressed air forced the hose free from its restraining rope and it whiplashed out violently, hitting Murray.

An HSE investigation found that Joseph Gallagher Ltd had failed to properly assess the risks involved with the cleaning process and the use of compressed air.

The workers had been given no safe system of work to adopt for the procedure.

Joseph Gallagher Ltd was fined £6,000 and ordered to pay £4,406 in costs after admitting safety breaches following the incident on 27 February 2010.

After the hearing, HSE Inspector Pete Collingwood said: “The case highlights the importance of producing a risk assessment and associated safe system of work for every dangerous activity.

“If this task had been properly planned and communicated to those ‘at the sharp end’ on site, it could have easily been prevented.”