The downgrade of a key investment security of Washington Trust Bancorp Inc., parent company of The Washington Trust Co. of Rhode Island, will cost the company $595,000 pre-tax, the company said.

The Washington Trust Co. has a small presence in New Bedford and Fall River.

The company said one of its perpetual preferred investment securities was downgraded to "below investment grade" after its 2008 earnings were announced, but before filing its 2008 annual report with the Securities and Exchange Commission. The after-tax charge to earnings will amount to $387,000, the company said.

"This charge will reduce full-year 2008 net income to $22.2 million, or $1.57 per diluted share, from $22.6 million, or $1.59 per diluted share, as previously reported," the company said in a statement. "For the fourth quarter 2008, this charge will decrease its previously reported net income to $4.2 million, or 27 cents per diluted share, from $4.6 million, or 29 cents per diluted share."