It is understood the figure relates to potential fines following a three-year investigation by the Competition & Markets Authority (CMA).
The Enquirer understands the level of possible penalties following the investigation into six companies were communicated to firms earlier this year.
But cartel busters at the CMA have remained tight-lipped about the case – refusing to even confirm which sector it was focused on.
They have also been uncharacteristically reluctant about publicising the results of such a major probe.
The investigation opened in March 2019 into possible breaches of the Competition Act in relation to the supply of construction services.
Keltbray said it is “addressing a civil regulatory matter relating to historical issues which arose under a previous management team.”
It added: “The timing and outcome of this matter is uncertain however the range of potential liability is between £3.9m and £16m.
“Having received advice a best estimate provision of £6m in relation to potential liability has been considered by the directors to be appropriate.”
Keltbray has also spent £500,000 in legal fees on the case.
Earlier this year Careys set aside £9m for “a regulatory matter relating to historical breaches due to the behaviour of a former management team”.
The CMA said the next official update on the case would be next month.
Keltbray’s results for the year to October 31 2021 saw a return to the black with an adjusted pre-tax profit before exceptional items of £5.7m compared to a loss of £6m last time from a turnover of £390m.
Darren James, Group Chief Executive said: “We are pleased to have delivered good results with strong order book growth, as we return to profit.
“We continue to make significant strategic progress to transition Keltbray into the UK’s leading specialist engineering and construction services provider.
“As a large and increasingly diverse engineering specialist, with a strong UK-wide brand and delivery footprint, we are well positioned to deliver on our core purpose to redefine the way sustainable development is delivered.”