The fire that raged for two days under central London streets was caused by an electrical fault within a Victorian service tunnel, investigators believe.
Initial conclusions from London Fire Brigade’s probe suggest the fault developed within cable linking thousands of Camden and Westminster properties to a local substation.
The incident worsened when the heat ruptured an eight-inch gas main, releasing gas that fuelled the blaze.
Investigators found that the tunnel, 3m underground, was well maintained and there was no sign of arson.
The alarm was raised at 12.39pm on Wednesday 1 April when smoke was seen rising from an inspection cover on a pavement in Holborn.
It took more than seven hours to get the blaze under control. Fire chiefs did not want to put the fire out until they had shut down the gas supply, as doing so could have led to a dangerous build up of gas in the tunnel.
Firefighters worked shifts to keep the fire under control, with 10 engines and 70 staff tackling the incident at its peak. The blaze was only declared extinguished on the morning of Friday 3 April.
While the London Fire Brigade will compile a full report on the incident, it is understood this is unlikely to be published. Although this incident was unusual in its scale, underground fires in London are not uncommon. Firefighters attend an underground fire in the capital about once a week.
“This technically difficult fire shows just how complex London can be and how unseen risks underneath the capital can significantly affect businesses, residents and the day-to-day running of parts of the capital,” said London fire commissioner Ron Dobson.
“Thankfully large underground fires like this one, which have such a wide impact, are very rare and seldom cause injuries.”
The fire left more than 3,000 homes and businesses without power, and UK Power Networks called in hundreds of in-house engineers for the restoration project.
“This is the biggest emergency cable replacement project that we as a company, have experienced to date - and its location beneath busy London streets brought extra challenges,” said a UK Power Networks spokesman.
“We restored the majority of the 3,100 electricity supplies affected on the day the fire started despite not having access to fire damaged cables in the tunnel.
“Our engineers have been working around the clock to lay a new electricity network to maintain the integrity of supplies to the wider area.
“At times we had over 400 engineers working on this, and more than 13km of new cabling needed to be brought to site. We are currently resurfacing some major road excavations and doing everything we can to get the essential repairs completed as quickly as possible.”
Restrictions to vehicle traffic on Kingsway, in Holborn remained in place on Thursday 9 April.
Camden and Westminster councils, the police, UK Power Networks and National Grid worked closely with London Fire Brigade on the incident.
The National Police Air Service helicopter used infra-red images to locate the main hot spots, providing valuable information for firefighters on the ground.
Fire chiefs sent the police’s explosives robot into the tunnel to gather more information.
“We discussed a number of plans, which included using high expansion foam, but as there was no compartmentation in the tunnels, there was no way of knowing where the foam would go and what structural damage it may have caused,” said Dobson.
“As the gas leak was fuelling the fire it was much safer to contain it while the escaping gas was burning off. If the fire had been put out before it was isolated it could have resulted in a build up of gas over a wide area leading to possible explosions.”