Shurgard, which runs the self-storage facility on Purley Way in Croydon, said it would contact all those renting units within the next two days.
The fire started at around 7.45pm on Monday and ripped through the warehouse. The cause is yet to be determined.
Nobody was injured, police said, although a spokesman for Shurgard said the building had been razed to the ground.
“The entire building has been destroyed,” he said. “Damage to the surrounding area seems to be limited, but for safety reasons traffic has been blocked for the time being.
“Shurgard representatives at the scene are working in close collaboration with the London Fire Brigade and the local police department to provide them with all necessary information on the store.
“The care for its customers of the 1,198 rented units at the Croydon Purley Way store is the first priority for Shurgard. Shurgard will undertake every effort to support all customers for which the event means a loss of their stored goods.
“As a first measure, a dedicated Shurgard team will be contacting every customer of the store within the next 48 hours to help them and support them with their claim procedures.”
London Fire Brigade (LFB) assistant commissioner Graham Ellis described the blaze as ”very challenging“.
He said: ”As you can imagine, self-storage units are generally full of items like furniture which when packed tightly provide a lot of materials to burn, and so these sorts of incidents tend to create large fires that burn hard for a long time, creating a lot of smoke.
“That said, firefighters have worked hard to ensure that while the fire is burning, it is contained and won't spread to nearby buildings.
“Currently, a thick plume of smoke is rising from the fire and while most of it is dissipating in the air, it is visible and locals will certainly smell it, so do keep windows and doors shut.
”Self-storage units are technically very difficult fires to manage, as well as the fire loading it's difficult to know what is being stored, which is another reason why we will have lots of appliances in the area for most of today.”
The road around the storage unit remains closed while emergency services continue to work at the scene.
At the height of the incident, 20 fire engines and around 120 firefighters were called to the scene to tackle the blaze.
The fire has now been contained, but LFB said fire crews were expected to be tackling “deep seated pockets of fire” in the building into Tuesday night.
Additional reporting by PA
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