Following a year of profit warnings, Bulmers saw a fifth of its value wiped off last week when it admitted it had unearthed a £3.3m black hole in its accounts.
The company has called in accountant Deloitte & Touche to investigate the error, which will come as a huge embarrassment to a management team alreay weighed down by a string of errors including a ill-fated foray into South Africa and over-zealous expansion in America.
The company's former accountant was Arthur Anderson.
Chief executive Mike Hughes said the costs, dating as far back as 1999, came to light during a business review instigated two months ago.
"We didn't need this," said Mr Hughes. "It is irritating and indeed concerning that there's another piece of bad news from Bulmers. Hopefully it will be the last bad news item."
Most of the costs relate to a Strongbow marketing push in supermarkets and cash-and-carry stores. The company spends about £70m in this market each year.
The shares fell 49p, or 21 per cent, in one day.
Analysts warned that the company, which makes Scrumpy Jack, Strongbow and Woodpecker, was in danger of losing shareholder confidence, permanently.