SACRAMENTO (CBS/AP) — State officials have stripped Blue Shield of California of its tax-exempt status, an exemption the nonprofit health insurer has held since its founding in 1939.

The California Franchise Tax Board quietly made the revocation last August.

The decision could put Blue Shield on the hook for tens of millions of dollars in state taxes each year. The insurer has paid federal taxes for years.

A spokeswoman for the tax agency declined to comment on the reasons the exemption was stripped. The highly unusual action comes after a lengthy state audit.

The newspaper says the move comes as the state’s third-largest health insurer faces fresh criticism over its rate hikes, executive pay and financial reserves, which in 2014 totaled $4.2 billion.

San Francisco-based Blue Shield is protesting the decision.

© Copyright 2015 The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten or redistributed.