Turnover at the Portakabin-owned business almost halved to £72m in 2025 from £140m the previous year.
Darwin plunged to a £5m operating loss from an £8.6m profit, while pre-tax performance swung from a £9.2m profit to a £3.2m loss.
Directors blamed the reversal on several large permanent modular healthcare projects which ran into operational delivery problems and cost overruns.
The setback has prompted Darwin to sharpen its focus on smaller, less complex modular buildings and the hire market as part of the wider Portakabin group.
The Telford-based business, which specialises in offsite healthcare construction, was acquired by Portakabin in 2023 as the modular giant pushed deeper into the NHS market.
Average employee numbers fell to 174 during the year from 200, with the contracts workforce dropping from 172 to 145.
Directors said the business would maintain its commercial focus on UK healthcare while concentrating on smaller modular sales and hired space.
