(Adds confirmation, background from sentencing, quotes)
WASHINGTON, Jan 27 (Reuters) - Republican U.S. Representative Trey Radel, who was charged in November with buying cocaine, plans to resign from Congress on Monday, his office said.
Radel, 37, elected to the U.S. House of Representatives in 2012 backed by the conservative Tea Party movement, returned to Congress in January, six weeks after he pleaded guilty to cocaine possession and checked into a rehabilitation clinic.
The Florida congressman plans to release a resignation letter on Monday, Radel spokesman Greg Dolan said.
Radel was charged with buying 3.5 grams of cocaine in Washington on Oct. 29, in the presence of an undercover agent.
The House Committee on Ethics in December said it would launch an investigation into whether Radel violated congressional rules or broke any other laws related to his responsibilities as a member of Congress.
Radel rebuffed calls for his resignation from the Republican Party of Florida and Governor Rick Scott as other Florida Republicans expressed interest in his seat.
"I look forward to getting back to work next week, representing my neighbors in Southwest Florida as they face the burdens of Obamacare, a jobless recovery, and a federal government that continues to spend more than it takes in," Radel said in a statement in early January.
The most recent posting on his Twitter account was from Jan. 3, when he said he took his son to his first movie. His last Facebook post, on Jan. 15, discusses draft legislation he was working on targeting identity theft.
The case against Radel stemmed from an investigation by FBI and the Drug Enforcement Administration agents into cocaine trafficking in the Washington area, Assistant U.S. Attorney Nihar Mohanty said.
Radel was sentenced to one year of probation in November.
"I am so sorry to be here. I know I have let my constituents down, my country down and, most importantly, my family, including my wife and my 2-year-old, who doesn't know it yet," Radel told Judge Robert Tignor after pleading guilty to the misdemeanor charge.
He said he would enter an intensive in-patient drug treatment program in Florida during his leave of absence. (Reporting by Susan Heavey and Doina Chiacu; Editing by Stephen Powell)
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