An "unacceptable number of complaints" were filed during the summer about the so-called Westbury Whiff, according to the Environment Agency.
The stink emanates from the Northacre Resource Recovery Centre - a mechanical and biological treatment plant that turns household waste into solid recovered fuel, which can then be used by renewable energy plants to generate power.
Hills Waste Solutions operates the facility under a 25-year contract for Wiltshire Council. It can handle up to 90,000 tonnes of waste per year.
The Environment Agency says the operator, Hills Waste Solutions, is "not currently taking all appropriate measures to reduce odour" and "remains in breach of its permit conditions."
Environment Agency officers monitoring odour reports say they have detected "very faint" or "faint" odours off-site, and in some non-residential areas, the intensity has been "distinct".
In an update to residents this week (September 24) the Environment Agency said it had given the operator until December 22 to implement further mitigation measures.
"We will continue to investigate odour reports and carry out scheduled odour rounds," the agency pledged.
A campaigner for improvement at the site has been Westbury town councillor Janet Parker.
Previously, Cllr Parker told the Times that although the stench was less intense, it has not gone away.
“It’s not as bad as it was and that’s probably because of the weather, but it is still ongoing and it is still causing quite a lot of misery for people,” she said.
"We are still encouraging residents to complain to the Environment Agency.”
Residents are urged to keep reporting odour incidents on 0800 80 70 60.
A spokesperson for Hills Waste Solutions said: "Hills Waste Solutions takes odour concerns very seriously and we continue to work closely with the Environment Agency to control odour at the Northacre Resource Recovery Centre.
"We have already upgraded the site’s biofilter and installed additional extraction equipment, and we are now undertaking a feasibility study to assess further improvements by 22 December 2025.
"We remain fully committed to working with the regulator and the local community."
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