Unscheduled flaring at the Mossmorran chemical plant in Fife is due to a compressor fault, Sepa has said.
Sepa said that it received reports of flaring at the ExxonMobil plant shortly after 3.30am on Sunday.
The government agency said it is gathering information to determine if there has been a breach of permit conditions.
In May, Sepa said it intended to seek prosecution over a series of unscheduled flares, which saw more than 900 complaints submitted.
Chris Dailly, head of environmental performance at Sepa, said: “Whilst it’s again important to stress that limited, controlled flaring is an authorised and important safety feature of industrial sites, we’ve heard clearly from over 380 reports to Sepa of the impact this further flaring event is having on local communities.
“Having referred ExxonMobil Chemical Limited to the Crown Office and Procurator Fiscal Service (COPFS) for consideration of prosecution related to Easter 2019 flaring, we remain frustrated by the frequency of flaring and the flow of information from the operator.
“We are investigating whether there has been a breach of permit conditions which would inform our next steps in line with our published enforcement policy.
“We are clear on our expectations of the operator, including requiring the installation of noise-reducing flare tips followed by the installation of ground flares.
“Moreover, the operator must take steps to make flaring the exception rather than routine and if they will not, Sepa will consider further options to ensure they do so.”
In a statement on Twitter, ExxonMobil wrote: “As our team undertakes the safe re-start of our compressor, you may see some fluctuations in our elevated flare tonight.
“We will continue to minimise the size of the flare wherever possible and apologise again for any frustration caused by this work.”
Fife Councillor Darren Watt said: “Once again, residents throughout the area are subjected to horrendous noise and light pollution because of yet another major operational failure by ExxonMobil.
“This is unacceptable and their predictable assurances are quite frankly meaningless.
“Actions ultimately speak louder than words and they consistently fail to alleviate people’s legitimate fears and concerns about the plants overall safety and viability.”
He added: “Furthermore, this latest malfunction comes just days after the future of Mossmorran was discussed at the Scottish Parliament and only strengthens critics argument that the plant is simply no longer fit for purpose and transition away from its current operations is absolutely paramount.
“Local communities aren’t asking for much. They just want ExxonMobil to be a more responsible and considerate neighbour but continue to be let down resulting in further erosion of trust.”