Mossmorran Ethylene plant in Fife is to face a joint investigation by the Scottish Environment Protection Agency (SEPA) and the Health and Safety Executive (HSE).
The announcement follows final written warnings served in April to operators ExxonMobil and Shell UK for prolonged unplanned flaring at the plant.
Operating permit variations are also to be issued to operators following a review to strengthen noise and vibration control, while flaring incidents in October, March and May flaring incidents remain under investigation and could still result in future enforcement action.
The plant has come under increasing pressure in recent months after scores of complaints from nearby residents over noise and light polution as well as health concerns.
Following a round-table meeting involving the operators, politicians, and community representatives, SEPA chief executive, Terry A’Hearn, told the Press: “We were clear that unplanned flaring in June 2017 was preventable and unacceptable and we issued final warnings to both companies in that regard.
“We’ve listened carefully to community calls for a root and branch review and the announcement of a joint investigation by SEPA and the HSE will examine issues at both sites.
“Operating permit variations will be served on ExxonMobil Chemical Ltd and Shell UK in the next week.
“These permits are legal documents which set out what each operator must do under law.
He added: “It’s right that people want to know more about October, March and May flaring incidents. People want action not words.
“We’re listening to the community and it’s important the companies do too.”
The news that a top-level inquiry will now take place has been welcomed by politicians.
Lesley Laird, Kirkcaldy and Cowdenbeath MP said the announcement was “a victory for residents.“
“It shows the power people have when we get all the right people in a room.
“There’s considerably more still to be achieved but now we have the attention and co-operation of all key agencies. We will continue in our endeavours working to secure the best positive outcomes for communities surrounding Mossmorran.”
David Torrance, Kirkcaldy MSP added: “A welcome update was given regarding planned future investment to upgrade the plant, help alleviate unplanned flaring and reduce the impact of flaring on our communities.
“Research into work to improve and redesign the flaring system through the use of the best technology available will also continue.”
Aspokesman for ExxonMobil said they would work with SEPA and HSE to ensure their operating scope is clearly defined and understood, added: “We are engaging positively with community representatives, local bodies and the regulators to address any concerns about our operations.
“We comply with all applicable laws and regulations, and fully recognise the importance of regulating the operation of a plant of the scale and complexity of FEP in order to minimise the impact on the surrounding communities and the wider environment.”