Welsh Slate was valued at £100m before the four-year fraud which led to a £40m drop in profits and the loss of 126 jobs.
Three former executives of a slate firm in north Wales have been sent to prison for inflating sales figures with more than £10m in false invoices.
Christopher Law, Geraint Roberts and Paul Harvey, admitted fraudulent trading at the former Alfred McAlpine slate division in Bethesda.
The Serious Fraud Office said that 44% of the firm's reported debtors were fiction.
They were sentenced at Caernarfon Crown Court for the four year fraud.
Law was jailed for 30 months, Roberts got 16 months and Harvey will serve 10.
Christopher Law (L) and Geraint Roberts admitted the charge
Law and Roberts have also been banned from becoming company directors for four years and three years respectively.
The crime was first uncovered by parent company Alfred McAlpine in February 2007.
Its audit team was asked to investigate why outstanding debts on the false invoices were not being collected.
Auditors then spoke to North Wales Police and a joint operation was held with the Serious Fraud Office (SFO).
It was found that customer letters were created to give the impression that payments were in the pipeline, while delivery notes and transportation invoices for non-existent consignments were forged, the SFO said.
The slate division was previously valued at £100m before the fraud which led to a £40m drop in profits and the loss of 126 jobs.
The slate business was sold for £31m at the end of the year, and now trades under new management. Parent company Alfred McAlpine was bought by construction rival Carillion in February 2008.
An example of the deception included showing auditors a stockpile of crates of roofing slate; the outer crates full but the inner crates empty.
Paul Michael Harvey was the sales director of the company
The SFO acknowledged that the purpose of the fraud was not to directly enrich the defendants but to convey to the main board of McAlpine that it was more successful than it was.
Following the case, historian and archaeologist Dr David Gwyn said the slate industry was costly with lots of competition.
"You could argue that the slate industry of Wales is the barometer of the building industry," he said.
"You could argue that the building industry is the barometer of the economy, and how much money there is in the economy is reflected in the amount of new build.
"So it is quite clear that the slate industry is faced with a very difficult period," he added.
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