Paramedics were called to a Lag BaOmer event in Stamford Hill on Wednesday evening, treating several people for minor burns after the blast on a residential street.
Video of the incident circulated online shows a large group of people crowded around waiting for the fire to be lit.
The bonfire, which was reportedly soaked in oil, erupts violently as a torch is used to ignite it, engulfing those standing closest in flames.
"It appears that the explosion was caused by fuel and not the smartphone, although there definitely were multiple smartphones placed inside the pile to be burned,"the report said.
London Fire Brigade said firefighters had spent roughly 15 minutes at the scene, but the cause of the explosion was not being investigated.