The District's high bond rating could fall if it continue to dip into cash reserves, the city's Chief Financial Officer Natwar Gandhi warned in a letter sent to Mayor Vince Gray and D.C. Council Chairman Kwame Brown on Thursday.

The letter follows a credit report issued last month by Standard and Poor's  that maintained the city's bond rating, but suggested it was in danger of losing its A+ rating if the city's savings account continued to drop.

"The District's financial positions was, in our opinion, good but we expect it to weaken based on a trend of using reserves to ffset revenue shortfalls," the S&P report said.

Four years ago, D.C. had $1.5 billion in reserve, but the city has repeatedly dipped into the extra cash as tax dollars have dried up during the recession. Now, the reserves stand at about $780 million.

"I am concerned," Gandhi said, "that if the District's GO bond rating were to be downgraded, the adverse impact to the District's reputation, both on Wall Street and particularly here in the Metro area, would be significant. We are surrounded by triple-A rating states and counties, and a downgrade would cause unwanted negative reaction from the press and, most importantly, from our residents."

It's not the first time Gandhi has issued such warnings. Last month, the D.C. Council passed a supplemental budget bill that not only closed a $188 million budget gap in the current fiscal year, but also set aside $40 million to prevent further dipping into the reserves. If the cash Gray put aside is not spent, it will be used to help fill an expected $500 million gap in the next fiscal year.