More than one-third of workers at a manufacturing site in Avonmouth which makes part for the new nuclear power station at Hinkley Point C have been diagnosed with Covid-19.
The Balfour Beatty site was shut down at the weekend after 22 of the 90 staff tested positive, but a further 14 staff have now become ill.
A deep clean of the site was carried out over the weekend - and NHS test and trace staff were alerted.
Balfour Beatty confirmed on 27 July there has been no impact on the construction site at Hinkley Point C.
A spokesperson said: "Having engaged with Public Health England and Bristol City Council we have now taken the responsible decision to close the facility until further notice to reduce the potential further spread of Covid-19.
"The NHS’ Test and Trace procedure has so far shown no impact on the Hinkley Point C main construction site, which the Avonmouth facility serves as part of the wider supply chain.
"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."
The spokesperson said enhanced hygiene and cleaning measures were in force at all its sites, with social distancing being observed and temperature checks being carried out on staff.