The number of workers to test positive for Covid-19 at a concrete-making plant in Avonmouth has increased to 28.
Balfour Beatty has closed the site which supplies the construction of the Hinkley Point C nuclear power station.
A second deep clean of the facility is set to take place this week after the first was carried out last weekend.
Christina Gray, director of Public Health for Bristol City Council, said measures in place would “prevent the outbreak spreading into the community”.
A further six positive tests were confirmed after the announcement of 22 cases on Monday.
Ms Gray said that all workers with positive tests are self-isolating and contact tracing is under way.
“Balfour Beatty reacted promptly, voluntarily closing its site to reduce the risk of infection and calling in public health experts.
“These actions, along with those we are now taking, will contain this outbreak and prevent it spreading into the community,” she said.
Balfour Beatty said it initiated the NHS Test and Trace procedure and that it had currently “shown no impact on the Hinkley Point C main construction site”.
“We continue to engage with all those affected and remain committed to the health, safety and wellbeing of all those who work with and for us, as well as the general public,” a spokesperson said.
The 28 positive tests in Avonmouth is more than the entire city of Bristol recorded in the past week.
Figures updated on Monday showed 18 new cases in the week to 23 July – about 3.9 per 100,000 people – up from eight the week before.
The new cases in Avonmouth will only be counted towards Bristol’s total if the workers live in the city.
Unite regional officer Heathcliffe Pettifer said the union had sought reassurances from Balfour Beatty over measures to prevent the risk of further transmissions once staff return to site.
“We are also calling on Balfour Beatty to ensure that all workers, regardless of whether they are directly employed by Balfour Beatty or a contractor, are paid in full while the site is closed,” he added.