Fairfax County Mayor Richard “Scott” Silverthorne was arrested and charged with felony distribution of methamphetamine and misdemeanor possession of drug paraphernalia on Aug. 4. (Reuters)

Fairfax City Mayor R. Scott Silverthorne announced Monday that he will resign after being arrested last week for allegedly seeking to trade methamphetamine for group sex.

Silverthorne, 50, will send a formal letter of resignation to the city Thursday, a day after he is scheduled to be arraigned in Fairfax County General District Court on a felony charge of distributing drugs, his attorney Brian Drummond said.

“That’s when he is going to go ahead and step down,” Drummond said.

Silverthorne, a Democrat, was recently elected to a third term as mayor of the Northern Virginia city of about 24,000 residents. Silverthorne, the son of former mayor Frederick W. Silverthorne, had previously served nine terms on the city council, starting when he was 24.

[Will Fairfax Mayor Scott Silverthorne face jail time after his methamphetamine bust? Maybe.]

Fairfax County police spokesman Major Ed O’Carroll described the circumstances surrounding the August 4 arrest of Fairfax City mayor Scott Silverthorne, in connection to an alleged methamphetamine-for-sex scheme. (WUSA9)

Silverthorne was arrested last Friday in an undercover sting operation set up by Fairfax County police detectives, during which Silverthorne allegedly handed an officer two grams of methamphetamine in a Crowne Plaza hotel parking lot in Tysons Corner with the expectation that the drugs were in exchange for participating in an orgy, officials said.

Two alleged accomplices — Caustin Lee McLaughlin and Juan Jose Fernandez — were also arrested in the sting operation, which police launched after receiving a tip in July that the mayor was involved with an illicit website.

Silverthorne — who was dismissed last week as a substitute teacher in Fairfax County after the charges were made public — allegedly sent a message to a fake profile police created on the site saying he could provide drugs for group sex.

[For small-city mayor, 2015 was a “terrible year.”]

His political downfall comes after Fairfax residents rallied around Silverthorne last year while he battled cancer of the neck and, after losing his job with a manufacturing trade association, dealt with a cascade of financial problems that included the loss of a five-bedroom house to foreclosure.

The mayor’s arrest has sent shock waves through the small city, where Silverthorne is known largely for his efforts to raise Fairfax’s economic stature by redeveloping old strip malls and other aging parcels of land in hopes of luring in new retail stores.

“I’m sad for Scott, I’m sad for his family and I’m sad for the city of Fairfax,” said former councilwoman Nancy Loftus, a political rival of Silverthorne’s who has known the mayor since they both attended elementary school in the city.

The city council has scheduled a special meeting Tuesday evening to deal with the effects of Silverthorne’s pending resignation, officials said Monday.

Under state law, the city is required to hold a special election to replace the mayor, though a date has not been designated.

Until then, council member Jeffrey C. Greenfield will serve as acting mayor, officials said.

Silverthorne officially appointed Greenfield to that post after being released on his own recognizance Friday in what was one of his last acts as mayor.