The firm linked-up with the private Corona Test Centre to test 231 people last week.
Full results shared with the Enquirer show 53% of the tests – which show if you have already had Covid-19 – came back positive.
Collins Construction director of health and safety Tony Turner said the high level of positive tests was a surprise because most people were unaware they had been ill or showed only minor symptoms.
He said: “Most cases showed no symptoms and people were unaware they had it.
“I think widespread testing is something that more contractors should do as having this sort of data can only help when it comes to protecting your staff and implementing new working practices.”
Collins is based in Croydon and works mainly across London.
Staff were offered tests free with a discounted rate for families and subcontractors.
Testing took place last week at 68 King William Street where Collins is refurbishing the 11-storey retail and office unit, previously occupied by House of Fraser, into a mixed use, office led building.
A second day of testing was then held for all staff at its head office.
The Corona Test Centre said its antibody blood tests are 98% accurate.
The decision to provide free testing comes on the back of a series of safety measures that Collins has implemented across all sites over the past 12 weeks.
These have included clear social-distancing guidelines, site-wide risk assessments and buddy systems to enable the same people work together on a daily and regular basis.