The firm at the centre of an underground fire at Corsham says it hopes be back to normal working by early next week.

The safety operation to clear up after the fire at a former mine could take until tomorrow night.

Yesterday afternoon, fire crews and mines safety experts began to pump liquid nitrogen into Leafield Logistics's underground complex at Monk's Park.

The aim is to starve the fire - which started 100ft under the ground early Wednesday morning - of oxygen.

The 18-acre underground complex is used to store defence supplies. Leafield Logistics supplies the Ministry of Defence and navies around the world with spare parts for ex-Royal Navy ships and submarines.

The affected chamber within the mine covers around five acres, and is secured by fire doors which are part of the fire management system and which emergency services say worked perfectly.

Leafield Logistics is operating a minimal service to its customers but commercial director Nigel Thomas said: “Obviously this has been a difficult couple of days, but the situation has always been under control and we are now moving towards this fire being put out.

"I would again emphasise that there is nothing stored in the affected area that carries any significant fire risk, and we have also been reassured by an asbestos expert that any smoke coming from the site does not contain asbestos particulates. We are keeping our neighbours updated, and hope to be back to working as normal early next week. “

A statement issued by the ambulance service and other NHS bodies yesterday said: "This afternoon, an operation started to pump liquid nitrogen into the affected chamber to help extinguish the fire. It is anticipated that this, together with follow-up observations and ‘settling’ time, will take until Saturday night at the earliest."

Some of the mines rescue team helping out in the operation have been drafted in from the Combe Down Mines in Bath, where stabilisation work is under way.

Health officials have advised people to stay 500 yards away from the site because of smoke.

A statement from the Health Protection Agency, NHS Wiltshire and Great Western Ambulance Service said: "This is precautionary advice and people living in the vicinity of the site are not at any risk."

The emergency services were called at 8.20am on Wednesday after the fire started in a forklift truck 80 to 100ft below ground.

Wiltshire Fire and Rescue Service said initial efforts by a team from the specialist company Mines Rescue - which is officially approved for such work by the Government - to get at the fire were thwarted because of "virtually non-existent visibility, due to the dark conditions and thick smoke."

Leafield’s own mines monitoring systems, which gauges heat and humidity, have been used together with Mines Rescue’s own monitoring systems to track the situation underground and the immediate area on the surface.

Once the liquid nitrogen operation is completed, the nitrogen will need to be left for a significant period of time to ensure that the fire is out and the atmosphere has cooled.

A fire service spokeswoman said: "It will be necessary to ventilate the chambers once the mines experts are satisfied that it is safe to do so, but this cannot happen until the weekend."