It added though that the involvement of top management in the accounting irregularities may have made it harder for auditors to detect problems, noting that the quality of the audit could only be determined by a separate investigation.
EY ShinNihon declined to comment for this article. Tanaka has said he never had any intention of encouraging accounting irregularities, but did not dispute the report's findings.
Some accounting experts say the scandal highlights the low audit fees paid by Japanese companies, which they believe are caused by historical caps, stiff competition and a corporate culture that does not value the audit function or shareholder transparency.
"One of the ongoing problems in Japan is that the fees paid by listed companies to auditors are very low compared to the international average," said Robert Medd, a partner at GMT Research in Hong Kong.
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Because accountants typically charge by the hour in Japan and elsewhere, fees can provide a rough proxy for the time spent on an audit, and can offer a comparable benchmark when measured as a proportion of a company's overall revenues.
An analysis by GMT of more than 2330 listed companies with $500 million or more in sales found Japanese firms pay their auditors on average 3.2 basis points of turnover, compared with 5.3 in the United Kingdom and 11.8 in the United States, the lowest among the major developed markets. The overall international average was 5.6 basis points.
Toshiba paid EY ShinNihon and other EY entities 1.5 basis points of turnover, or 982 million yen ($8 million), to audit its books for financial year ended March 2014, according to its financial statements.
The six-year average at Toshiba was 1.8, a Reuters analysis of the company's financial statements shows.
In a statement, Toshiba said EY's remuneration was "appropriate" and noted that audit fees vary each year due to one-off events. "As a listed company, we recognize that it is not whether the fees are large or small, but that it is important to receive the necessary and sufficient audit."
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A London-based spokeswoman for EY and the two audit oversight divisions of Japan's main regulator the Financial Services Agency all declined to comment.
A spokesman for the Japan Institute of Certified Public Accountants, the self-regulatory body for accountants, said it will investigate EY ShinNihon's involvement in the case.
To be sure, fees are not a water-tight gauge of audit rigor or competence given some companies can receive discounts, some sectors are less complicated to audit than others, labor costs vary across markets, and accounting rules in some countries can be less onerous.
Audit fees in Japan were low due to historical regulatory caps and although these limits were removed more than a decade ago, fees have struggled to reach developed-market norms, which accountants say can impinge on quality.
"In these highly competitive fee environments, it is always a battle to keep up the consistency of detailed service," said Chris Devonshire-Ellis, chairman of accountancy firm Dezan Shira & Associates.
Japan's government has tightened audit regulation, but low fees and overwork make it tough for firms to attract and retain quality staff, said Yoshinori Kawamura, professor in the Faculty of Commerce, Waseda University, specializing in accounting.
"The number of experienced certified public accountants is limited. Each senior auditor signs the auditor's reports for a good number of companies," he said.