The underground fire that caused a mass power outage in central London was caused by an electrical fault, London Fire Brigade (LFB) has said.
Investigators said it was believed the fire started due to a "fault in the Victorian tunnels".
They found the electrical fault damaged an eight inch gas main, which ruptured, fuelling the fire.
Some 5,000 people were evacuated during the blaze in Holborn on 1 April. It took 36 hours to put out.
Investigators said the fire, which caused thousands of properties to lose their gas and electric supplies, was not started deliberately.
London fire commissioner Ron Dobson said: "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.
"Thankfully, large underground fires like this one, which have such a wide impact, are very rare and seldom cause injuries."
As the fire burned, LFB said it used the Metropolitan Police helicopter and infra-red images to locate where the main hot spots were and used Scotland Yard's explosives robot to go into the tunnel to provide information.
Mr Dobson said: "If the fire had not 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."
During the blaze, eight theatres were forced to cancel performances and numerous restaurants had to close their doors.
Restrictions to traffic remain in place along both carriageways.