The alarm is raised and warehouse staff are evacuated. A malfunction of the site's cold storage facility has caused the dangerous chemical ammonia to leak - omitting a corrosive vapour that eats away at tissue on contact.

After the roll call, it becomes clear one person is unaccounted for.

This was the scenario Kent Fire and Rescue Service crews had to face yesterday at Lidl Distribution Centre in Northfleet, as part of a specialist training exercise set in a real-life working environment.

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VIDEO: Watch the dramatic chemical spill training exercise carried out by Kent Fire and Rescue Service

Firefighters from Gravesend and Swanscombe were tasked with carrying out a snatch rescue of a casualty overcome by the toxic fumes. Once safely outside, two men wearing large green specialist chemical protection suits made their way inside to identify the substance.

"It was hot, arduous, and more difficult with the fogging up of the gas-tight suit when you're looking for small leaks", said Ian Cleary, one of the firefighters donning the specialist clothing.

The risk of ammonia causing cold burns meant Ian had to wear his heavy firefighting gear underneath the suit.

"It's really hard to move," he added. "As we exhale the suit will fill up with our expelled oxygen.

"So every so often, you need to expel the air from the suit.

"You're constantly working in twos. Communication is very difficult. You almost end up using sign language."

"It puts us to the test, and lets us have the experience of wearing kit that we probably wouldn't wear every day."

While the casualty was rescued and the toxic fumes analysed, other crew members set up an exclusion zone - blocking drains with special mats to prevent the leak from contaminating the water supply.

Aside from all the action, it was a normal working day for Lidl warehouse staff. Some employees even got involved in the exercise, reenacting their emergency response plans to ensure they are prepared for such an incident.

Lidl's Head of Logistics, Stacy Copley, said: "To be effective everyone needs to understand what their role is in an emergency situation.

"Carrying out this training exercise with Kent Fire and Rescue Service is a valuable experience in preparing us to implement the plan in the unlikely event of an incident of this nature occurring."

Speaking after the exercise had finished, Leanne McMahon, West group manager, said the simulation was a success.

"It's gone really well. We've been able to use specialist advice and equipment that the crews wouldn't have otherwise been able to use.

"We're known a lot for attending road traffic collisions and fires, and these situations with chemicals are more rare. But we do need to be prepared for them."