U.S. Senator to FBI -- An IMPOSTOR Sold D.C. Internship to 'Girls Gone Wild'

U.S. Senator Mark Pryor is adamant ... he did NOT agree to auction off the 4-week congressional internship allegedly purchased by Joe Francis ... and has told the FBI an IMPOSTOR is to blame.

Here's the back story -- Francis sent out a press release Tuesday claiming he won an internship in a charity auction ... and plans to gift the internship to the winner of his HDNet reality series, "The Search for the Hottest Girl in America" (pics below).

Francis claims the internship will take place this summer in the office of U.S. Senator Mark Pryor (D – Ark.) in Washington, D.C. According to the auction website, Francis paid $2,500 for the prize.

Problem is ... Pryor is now calling B.S. ... and a rep for the Senator tells TMZ, "Our office does not sell, auction or donate internships."

"We believe someone outside Senator Pryor’s office has broken the law by fraudulently impersonating a U.S. Senator, fraudulently attempting to sell a government position and using the Senate seal without authorization."

"We have asked the FBI to fully investigate who is perpetrating this fraud against the senator and the U.S. Senate."

Francis -- who has not been accused of any wrongdoing -- tells TMZ, "My only intention in purchasing this auction item was to support a local non-profit charity."

He adds, "At all times I believed in the legitimacy of this internship."

The people behind the auction had no comment.