Hygiene Solutions, which is headquartered in King's Lynn, produces Deprox machines which pump hydrogen peroxide vapour on to sealed and empty wards to clean them.

It has been at the centre of a storm, with NHS workers reporting that when they entered a room to operate the machines their eyes would begin to sting. Others reported chest infections and nose bleeds.

MORE: Discount store in Castle Mall set to closeBut Hygiene Solutions hit back, saying the product was safe and effective "when used correctly".

The Health and Safety Executive has concluded a four-year investigation into the Deprox machines, and has told Hygiene Solutions to tighten up its safety measures.

The firm has been ordered to either provide hydrogen peroxide monitoring equipment or ensure that its customers are fully aware of the associated risks should they not use one.

A spokesman for the company said that Deprox was a "ground-breaking technology that when used correctly has been proven to be safe and effective at reducing infections in hospitals".

The spokesman added: "Deprox is designed to provide high-level disinfection to hospital environments to assist in managing outbreaks and reduce the risk of the spread of infection to patients.

"As with all similar products, the operator instructions and the equipment carry warnings about their safe use and it is important that the operator must perform the required steps correctly when using the equipment.

"Hygiene Solutions provides operator training, support and advice to hospitals using our technologies as part of our commitment to them and their staff."

The NHS is now using an alternative method of decontamination - but a freedom of information disclosure revealed that at least £2.1m was spent on the machines.

This figure is likely to be higher as many NHS trusts refused to reveal how much they spent on the machines, saying it was "commercially sensitive" information.