WASHINGTON — Rep. Trey Radel plans to resign, two months after the Florida Republican pleaded guilty to drug possession, prompting a congressional investigation and primary challenges in his district.

Radel, 37, planned to tell House Speaker John A. Boehner, R-Ohio, of his decision in a letter today, a spokesman said.

The first-term congressman, a former radio and television host in the southwest Florida area he represents, agreed to one year of probation after pleading guilty in November to a misdemeanor charge of possession of cocaine.

Radel had come to the attention of the FBI and the Drug Enforcement Administration during an investigation of drug trafficking in the Washington area. The congressman purchased 3.5 grams of cocaine from an undercover officer during an October “buy and bust” sting.

After his plea, Radel took a leave of absence from the House to begin undergoing treatment for what he said was a struggle with alcoholism. He returned to the Capitol this month, but at the time did not address his future, saying a possible reelection bid was “the absolute last thing on my mind.”

The House Ethics Committee created an investigative subcommittee in December to determine whether any of Radel’s actions violated the chamber’s rules.