Ineos has been warned over flaring at its Grangemouth chemical plant.
The company said an “unplanned event” had occurred at the facility in Falkirk but refused to explain further.
It resulted in the plant burning “off-spec” gas in a process known as flaring.
Noise from the flare could reportedly be heard several miles away from the plant on Thursday night.
Elaine Stewart was able to film it from Station Road in Polmont, more than a mile and a half from Grangemouth.
On Friday, the Scottish Environment Protection Agency said it was aware of the situation and warned Ineos that complying with environmental standards is “non-negotiable”.
Ineos said it was investigating the cause of the “unplanned event” and apologised.
“The flaring that resulted from the unplanned event has now concluded,” a spokesman said.
“We are undertaking a thorough investigation and we appreciate the disturbance that our operations caused.”
People living nearby have repeatedly raised concerns about flaring at Grangemouth, raising fears about its impact on air quality, as well as the noise and vibration it causes.