NEW YORK (Reuters) ? PricewaterhouseCoopers? lucrative, 30-year relationship as American International Group Inc.?s auditor is under threat amid the cascading evidence of accounting improprieties at the embattled insurance powerhouse, experts said.

Not only is AIG being subpoenaed by regulators over the accounting blow-up but it is likely to face investor wrath if it is proved that PWC auditors made some blatant errors while auditing the company?s books.

On Wednesday, AIG acknowledged bookkeeping mistakes that could stretch back 14 years and included improper accounting of reinsurance transactions and setting up offshore units. The disclosures raise the possibility of AIG reducing the value of its capital by around $1.7 billion.

Now, the Securities and Exchange Commission has subpoenaed the audit firm as authorities investigate accounting at the insurance giant.

As the improprieties unravel, accounting experts say, it would increasingly point toward substandard auditing done by PWC, which accounting experts contend did not ask enough questions while going through AIG?s various transactions.

?It was a case of weak auditing,? said Partha Mohanram, accounting professor Columbia University.

Recently, very few auditor-client relationships have survived after the auditor was found to have conducted substandard audit work.

At the heart of AIG?s problems are two issues. First, the way it treated a deal with Warren Buffett?s Berkshire Hathaway Inc. in which income should have been classified as a deposit rather than insurance income.

Second is its relationship with certain offshore vehicles which were deemed as independent from AIG but now seem they could potentially be considered part of AIG.