A total of 64 people have tested positive for coronavirus at the Iceland supermarket distribution centre in Swindon.
The borough council confirmed a further 13 staff had tested positive at the site run by XPO Logistics on Friday.
The Unite Union says some employees, 150 of whom are self-isolating, are now afraid to return to the premises.
A spokesman for the company said it is working with local authorities in an attempt to keep staff safe.
Swindon Borough Council says that 457 of the site's 750 employees have now been tested.
A mobile testing unit was set up at the depot on 29 July and the 150 staff members currently self-isolating are doing so as part of test and trace measures.
The council's Director of Public Health Steve Maddern said of the positive cases so far, two were being treated in hospital.
"Inevitably, the number of cases has gradually increased this week, which can be expected as a result of the extensive onsite staff testing.
"We are now seeing the number of new cases starting to reduce as the outbreak is contained," he said.
Mr Maddern added that the risk of the outbreak to the wider community "remains low".
John McGookin of the Unite Union said employees were fearful of returning to work and felt they had been kept in the dark.
"Let's be honest, if you've had the number of cases that this site has had in a small town, you'd be seriously thinking of quarantining the site," he said.
"We have no idea why they [XPO Logistics] have not done that. They are not communicating with us."
A spokesman for XPO Logistics said: "Protecting our employees is our number one priority.
"We're working closely with the local authorities and adhering to all local and Public Health England guidance to ensure the appropriate health and safety measures are in place."