Dave Natonski says the freshman Republican will step down Monday and will send a letter to House Speaker John Boehner.
Politico first reported the upcoming resignation.
On Nov. 20, Radel pleaded guilty to a misdemeanor charge of cocaine possession and was sentenced to a year of probation. He admitted to purchasing 3.5 grams of cocaine from an undercover officer Oct. 29 in Washington.
Several GOP leaders had asked him to resign. But Radel had pledged to stay in office and rebuild constituents' trust after completing in-patient treatment for drug and alcohol abuse.
Radel had been in office for 10 months when charged. His district includes the Gulf Coast cities of Fort Myers and Naples.
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