Rep. Trey Radel, a Florida Republican serving in his first term, resigned today, bowing to building pressure from House Republican leaders to step down after he pleaded guilty to a cocaine possession charge.

"Unfortunately, some of my struggles had serious consequences," Radel wrote in a letter announcing his resignation Monday morning. "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, Southwest Florida."

Radel was the target of an undercover sting operation when he was busted for possession of cocaine Oct 29. He pleaded guilty in a Washington, D.C. Superior Court and was placed on one year's probation with "minimal supervision."

After returning from rehab, House Speaker John Boehner and other Republican leaders quietly encouraged Radel to resign, according to sources with knowledge of the discussions. Publicly, Boehner had said the decision to serve was one for Radel, his family and his constituents to make.

"It was pretty clear he was on his own on this one and wasn't going to be hugged by leadership," one top Republican source said.

Today, Radel thanked Boehner and his colleagues in Congress for "tremendous support and encouragement."

"Oftentimes in Congress, our personal relationships and successes are overshadowed by intense but meaningful and necessary debate. However, I leave the House of Representatives with friendships and memories of great men and women dedicated to helping and improving the lives of our fellow Americans."

Radel left rehab last month, returning to Congress briefly this year following his arrest. At the time, he insisted to reporters then that he would not resign.

"No one will take away my passion when it comes to serving southwest Florida," he said at a Dec. 19 news conference.

In his letter of resignation, which he also sent to Florida Gov. Rick Scott, Radel added that despite "some personal struggles in 2013," this year "has already been tremendously positive as I focus on my health, family and faith," leaving the door open to a future run for public office.

"As an eternal optimist, I know there are great things in store for our country when we find ways to work together," Radel wrote. "Whether it is as a father, a husband, or in any future endeavor, I hope to contribute what I can to better our country in the years to come."

The House Ethics Committee had established an investigative subcommittee Dec. 16 to determine whether Radel's criminal actions violated the Code of Official Conduct. It's unclear whether that investigation effectively ends once Radel's resignation becomes effective at 6:30 p.m. today since the committee may continue its probe at its own discretion.

Tom Rust, the Interim Chief Counsel & Staff Director at the committee, said it does not comment on any particular matter unless it releases a public statement, which it has not for Radel.

The last time a Member was charged and convicted of criminal conduct and later resigned, the matter of Representative Jesse Jackson, Jr., a committee report from the end of the 112 th Congress noted an open investigation into Jackson, but "Representative Jackson resigned from the House on November 21, 2012, and the Committee no longer has jurisdiction over him."

On the other hand, when former New York Democratic Rep. Eric Massa resigned in March 2010 after allegations surfaced that he had groped two staffers, the Ethics committee continued its probe well beyond his resignation.

A special election will be held to fill the Radel seat, according to Florida statute.

ABC News' Steven Portnoy and Jeff Zeleny contributed to this report