A "pungent fishy stink" wafting from a creamery during the night is making people feel sick and keeping them awake, say residents in north Cornwall.
The smell has also been described by those living near the Davidstow plant as like cheesy sewage.
The Environment Agency said the smell was coming from the waste-water treatment plant and ordered the firm to reduce odour and noise pollution.
Dairy Crest said it was working to cut its environmental impact.
Andrew McKersie, who is part of an action group complaining about the odour, said: "The worst thing is that we typically seem to get the smell in the middle of the night.
"We're in bed and suddenly you are woken up by the stink, and then you can't get back to sleep, so you can't sleep properly."
Phil Potter, who lives about a quarter of a mile (400m) away from the treatment plant, said he was not able to go outside during the summer months because of the smell.
"As soon as that smell came over you'd just end up feeling sick," he said.
The Environment Agency said an investigation found Dairy Crest "remains non-compliant with its environmental permit".
It issued an improvement notice to the company.
The agency said steps were being taken by Dairy Crest to "maintain appropriate conditions" in reception tanks at the waste-water treatment plant.
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A Dairy Crest spokesman said "recent improvements" at its waste-water treatment plant were "already starting to make a real difference".
"We are working closely with the Environment Agency to address the concerns that have been raised," he said.