A former Prince George's County Council aide testified yesterday that someone forged his signature on a letter that council member Anthony J. Cicoria and his wife used in obtaining a mortgage to buy an office condominium.

Testifying in Cicoria's theft trial, the former staff director, Samuel Wynkoop, said he did not sign the letter, which stated the county would lease the condominium from the Cicorias, and said such an arrangement would violate the county charter.

The letter helped the Cicorias finance the purchase of the condominium in the Prince George's Plaza Professional Building near Hyattsville. The county never leased space there.

Maryland prosecutors allege that Cicoria, 50, a second-term Democrat, bought the condominium with his wife as a personal investment, set it up as a constituent services office, then illegally used $33,000 in campaign contributions to make mortgage and property tax payments.

Cicoria, who faces Republican J. Lee Ball Jr. in the Nov. 6 general election, contends that the payments were proper, arguing that the office was used for campaign-related activities. A conviction would not necessarily preclude Cicoria from beginning a third term in January if he is reelected.

In applying for the $104,000 mortgage in late 1986, the Cicorias allegedly presented Dominion Federal Savings and Loan, of McLean, with a letter bearing Wynkoop's signed name. The letter, typed on Wynkoop's office stationery, stated that the Prince George's government had agreed to lease the condominium from the couple for four years, starting at $750 a month.

Wynkoop, who no longer works for the county, was shown the letter yesterday by prosecutor Scott Nevin, and described it as fraudulent.

He said the signature was not his. He also noted that the letter's date, Nov. 11, 1986, had been typed at the top of the page. He said it was the practice in his office to stamp dates on letters rather than type them.

Wynkoop said the Prince George's charter requires authorization from the County Council before the government can agree to a lease lasting more than one year. No such approval was sought, Wynkoop said.

In his opening statement last week, Nevin told the Circuit Court jury that the charter forbids the county to enter into a contract with employees, including council members.

Under cross-examination by Cicoria's attorney, Robert Mance, Wynkoop agreed that many employees in the council offices had access to his stationery, and that someone other than Cicoria could have written the letter. Mance did not question Wynkoop about Cicoria's alleged use of the letter in obtaining the loan.

The office condominium was purchased by Cicoria and his wife, Catherine, for $130,000. The couple allegedly used campaign money to make the monthly $922 mortgage payments, as well as tax payments, from early 1987 until they were indicted on theft and conspiracy charges last fall.

Dominion Federal, which changed its name to Trustbank Savings, filed a foreclosure notice in Circuit Court two weeks ago. Although the lender did not say in court documents why it was foreclosing on the property, it did allege that the Cicorias owe nearly $102,000 in principal and interest.

In all, Cicoria and his wife are accused of siphoning $64,324 from his campaign fund in the 1980s.

The council member is charged with three counts of filing false income tax returns and one count each of theft and conspiracy. Catherine Cicoria, who is her husband's campaign chairman, faces the same charges, plus seven counts of perjury for allegedly signing false campaign finance reports.

Catherine Cicoria, 52, was to have gone on trial with her husband. She did not show up for the start of the trial, cannot be located and is being sought by authorities.