WASHINGTON (AFP) - Trey Radel, the US congressman who pleaded guilty to cocaine possession last year, resigned on Monday, saying he could no longer effectively serve his constituents.

The first-term Republican announced his resignation in a letter to House Speaker John Boehner, acknowledging "some personal struggles" last year.

"Unfortunately, some of my struggles had serious consequences," Radel wrote. "While I have dealt with those issues on a personal level, it is my belief that professionally I cannot fully and effectively serve as a United States Representative to the place I love and call home, South-west Florida."

Radel, 37, was arrested in October after he was caught purchasing 3.5 grams of cocaine from an undercover police officer outside a Washington restaurant. He faced a maximum penalty of 180 days in jail, but instead was sentenced to one year of supervisory probation.

Radel spent nearly a month at a rehabilitation facility. After his leave of absence, he had said he aimed to continue serving in the House of Representatives.

Radel, a former radio talk show host and newspaper owner, came under pressure from fellow Republicans, including Florida Governor Rick Scott, to step down, but for a few months he stood firm.

"Sometimes in life you need a wakeup call. This is my wakeup call," he said in November before undergoing rehabilitation.

Radel's term ends in January next year, and he would have stood for re-election this November. His departure triggers a scramble for the open seat, with speculation that Radel's predecessor, Republican Connie Mack, who held the position before an unsuccessful run for the US Senate, might seek to reclaim the seat.

In the interim, Mr Scott is planning to schedule a special election to fill Radel's seat for the remainder of his term, the Miami Herald reported.

Radel, in his letter, noted the inflammatory partisan feuding in recent years, saying recent successes in Congress have been "overshadowed by intense but meaningful and necessary debate".