Jesse Jackson Jr. announced his resignation after serving in a Chicago-area district since 1995. (chicagopublicmedia/Creative Commons)

Jackson is departing from office for health reasons after being on medical leave since mid-June. He has been treated twice at the Mayo Clinic for bipolar disorder.

The 47-year-old eldest son of the civil rights leader was also facing a U.S. Justice Department investigation over allegations that he used campaign funds to redecorate his house and buy an expensive watch for a friend.

Jackson admitted to some of his mistakes in a letter to House Speaker John Boehner.

"I am aware of the ongoing federal investigation into my activities and I am doing my best to address the situation responsibly, cooperate with the investigators, and accept responsibility for my mistakes, for they are my mistakes and mine alone," Jackson said.

Jackson's resignation will lead to a special election for the House seat located in a heavily Democratic district. Illinois state law mandates that the election be held within 115 days after the seat is vacated.

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