Ten fire engines and around 70 firefighters were called to a fire at a commercial property on Cornhill in the City of London.
The fire occurred within a room in the basement of a seven-storey building consisting of offices above ground level. There have been no reports of any injuries.
Station Commander Duncan Rees, who attended the scene, said: "Due to the location of the fire, which was in the basement of the property, extra resources were called to the scene early on in the incident as a precaution.
"However, firefighters worked efficiently to bring the fire under control promptly and prevent the fire spreading further into the building. Crews remain on scene damping down hotspots.
"Currently, Cornhill is closed and we would ask people to avoid the area where possible, although we expect our presence at the scene to reduce significantly as we head into rush hour."
The Brigade's Control Officers were alerted to the fire due to a smoke alarm that started sounding.
A London Fire Brigade spokesperson said: "This incident highlights the importance of working smoke alarms, particularly if a fire occurs at a time of day when people are unlikely to be present to report it directly to us.
"Smoke alarms give the earliest possible warning when a fire starts and we urge everyone to make sure they have one fitted in every room where a fire can start except kitchens or bathrooms where heat alarms are more appropriate."
The Brigade was called at 0334 and Control Officers mobilised crews from Dowgate, Whitechapel, Shoreditch, Dockhead and surrounding fire stations to the scene. The fire was brought under control by 0452. A32-metre turntable ladder from Soho Fire Station also attended and was used as an observation tower to provide the Incident Commander with a greater situational awareness of the incident.
The fire is being treated as accidental and is believed to have been caused by self-heating textiles.
A London Fire Brigade spokesperson said: "These types of fires often involve textiles that become contaminated with oil, most commonly linseed, massage and cooking oil on tea towels, tablecloths and chef whites. Make sure you check the manufacturer's cleaning recommendations of your textiles before washing.
"Sometimes when materials are cleaned and put in tumble dryers, the heat from the tumble drying cannot escape. This can result in a high enough temperature allowing it to build up to a point where it smoulders and eventually ignites.
"If you are washing and then drying on a hot cycle, always use the cooling cycle on the tumble dryer so it cools down and allow the heat to dissipate properly, before stacking laundered items together.”