Five workers at the Hinkley Point C construction site in Somerset have tested positive for coronavirus.
Unite the Union says workers from different companies on the site have contracted the virus.
They include one person from Balfour Beatty, one from Bylor, one from G4S FM and two from catering company Somerset Larder - who have been providing food to the NHS.
A spokesman for the union has confirmed the most recent case was recorded on Wednesday April 29.
Workers thought to be at the highest risk of exposure to the infection have had their security passes deactivated and are not allowed back on site unless they test negative for the disease.
EDF, the French company behind the multi-billion pound project, has said there is 'no evidence of infections on site caused by the other workers'.
A spokesman for the company said that since the beginning of the pandemic five members of staff on the Hinkley Point C site have tested positive, one of whom hadn't been on site for weeks before becoming unwell.
They added that investigations are being carried out to reduce the risk of spreading infection, including contact tracing and monitoring the health of employees who are in, or have been in, self-isolation.
When workers and local residents raised concerns over the apparent lack of social distancing measures in place, Hinkley Point cut down staff numbers and introduced policies to make sure government guidance could be adhered to.
Hinkley Point C, the largest construction site in Europe, slashed the number of staff allowed on the ground by more than 50% to less than 2,500.
Bosses at the site say workers are now 'well adapted' to progressing with the project while adhering to social distancing regulations.
Jobs at all levels are being done from home where possible, and those who need to be on site are being given extra protective provisions.
See how Hinkley Point C has changed its way of working in their statement below:
At Hinkley Point C, our remaining workforce is now well adapted to social distancing and the experience gained has helped us to make a small and safe increase in numbers to just under 2,500, still fewer than half what it was at the start of the year.