Building safety provisions and contract losses due to cost inflation, labour shortages and subcontract failures combined to see Bouygues UK sink to a £42m loss last year.
The firm, which works on an array of projects from HS2 and Hinkley Point C to major hospitals and housing projects, also reported turnover down by a fifth to £383m in the year to the end of 2022.
The hit saw French parent company inject £47m into the business last December.
Among a £21m bundle of provisions the firm set aside £5.2m for contracts, around £1m for staff restructuring over the year and nearly £15m for building safety works.
Despite the fall in workloads head count over the year rose by around 12% to nearly 2,000.
The results were published at Companies House just two weeks after Bouygues UK Chief Executive Officer Rob Bradley announced he was retiring from the business as part of a restructure.
He has been succeeded by Fabienne Viala as CEO, in addition to her role as Chair and UK Country Director for Bouygues Construction.
The latest restructure sees Bouygues UK now organised into three business units.
Two of these will be regional businesses:
London & South-East, led by Philippa Prongué who is taking on a wider role since joining the business in May 2022 and who will bring the Contracting and Property Development teams together;
South West & Wales, led by John Boughton who will focus particularly on developing the regional business through frameworks;
The third business unit, Complex Projects, will be led by Gerald Farque, and will primarily concentrate on healthcare, science, and research projects nationwide.