Two senior managers at a Gwynedd slate firm have been suspended amid an internal inquiry into financial irregularities which could exceed £24m.
Accountants are being drafted in to analyse records
In a statement Alfred McAlpine Plc said it had uncovered a "systematic misrepresentation of production volumes and sales for a number of years".
The inquiry involves a "number of senior managers" at its slate subsidiary at Penrhyn Quarry, Bethesda.
North Wales Police are not believed to be involved.
Accounts dating back to 2003 will now be examined.
The inquiry was launched following an internal audit team investigation into the accounts at the site.
The company said it believed that accounts of production volumes and sales figures were exaggerated by a figure which could amount to £24m.
But it warned that it would "continue to have a material impact on the profitability and cash generation" of the slate business in 2007.
Auditors discovered that "those involved sought to conceal the financial implications of their actions through the pre-selling of slate at substantially discounted prices."
The statement added: "Whilst investigations are currently at a preliminary stage, the board currently believes that the financial implications of these accounting issues will involve a restatement of the 2005 net assets of the business by approximately £11m."
The company also said it would lead to a "reduction in the expected pre-tax profit of the slate business in the year to 31 December 2006 by approximately £13m, prior to any restructuring and impairment provisions that may be required in these accounts."