London Fire Brigade Deputy Commissioner Jonathan Smith said: "At 23:23 on the evening of 20 March, London Fire Brigade received the first of 212 calls to a fire in a high voltage substation near Heathrow Airport.
"The fire involved a transformer comprising 25,000 litres of its cooling oil fully alight. This created a major hazard owing to the still live high voltage equipment and the nature of an oil fuelled fire. As of the most recent update, approximately 5% remains alight in isolated hotspots, and we will maintain a presence at the site until the incident is resolved.
"The first appliance was on scene within five minutes of mobilisation. LFB was in close contact with blue light partners overnight, and at 00:42 the Metropolitan Police Service (MPS) declared a major incident.
"Ten fire engines, two Bulk Foam Units and one High Volume Pump were on scene at the peak of the incident – this equates to approximately 70 LFB personnel.
"Our firefighters worked tirelessly in challenging and hazardous conditions overnight to bring the fire under control as swiftly as possible.
"Firefighters safely evacuated 29 people from neighbouring properties, and as a precaution, a 200-metre cordon was established. Working with MPS, around 150 people were evacuated to a rest centre. The majority of those people have been able to return to their homes and very few people remain in the rest centre.
"London Fire Brigade Fire Investigation Officers, supported by a scientific advisor, have been working closely with the Metropolitan Police Service (MPS) to investigate the cause of this fire. The MPS has confirmed that the fire is believed to be non-suspicious. LFB’s investigation will now focus on the electrical distribution equipment.
"The Brigade is the enforcing authority for the Regulatory Reform (Fire Safety) Order 2005 and has therefore had Fire Safety Officers at the scene to follow up various lines of enquiry, and work will continue into the coming weeks.
"I would like to take this opportunity to thank Firefighters and Control officers for their courage and professionalism in bringing this incident under control in what were very challenging circumstances."