Illinois Rep. Jesse Jackson Jr. resigned Wednesday from Congress amid a cloud of health and legal problems that have intensified in recent months.

The 10-term Democrat left Washington unexpectedly in June, leaving fellow Capitol Hill lawmakers, his staff and voters to learn several weeks later that he was being treated for a bipolar disorder.

In Jackson’s resignation letter to House Speaker John Boehner, he cited both health problems and a federal investigation.

“For seventeen years I have given 100 percent of my time, energy and life to public service," Jackson wrote. "However, over the past several months, as my health has deteriorated, my ability to serve the constituents of my district has continued to diminish. I am aware of the ongoing federal investigation into my activities and I am doing my best to address the situation responsibly, cooperate with the investigators, and accept responsibility for my mistakes, for they are my mistakes and mine alone.”

A Boehner spokesman confirmed the letter had been received on Capitol Hill.

The 47-year-old Jackson will be replaced in a special election in the state's 2nd Congressional District.

Democratic Gov. Pat Quinn said Wednesday he will announce a schedule within five days for a special election.

However, Cook County Clerk David Orr told Fox News the election will likely be held on the standard election days in either February or on April 9.

Jackson, the son of civil rights leader Jesse Jackson, was easily re-elected earlier this month despite not having appeared in public since this summer. He had several visits in recent months to the Mayo Clinic for treatment.

The federal investigation is purportedly focusing on whether Jackson misused campaign money.

In addition, the House Ethics Committee has been looking into whether Jackson was involved in discussions about raising money for imprisoned former Illinois Gov. Rod Blagojevich's campaign in exchange for an appointment to President Obama's vacated Senate seat.

Jackson’s resignation would effectively end the House probe, though members will issue a final report.

“His service in Congress was marked by his eloquent advocacy for his constituents’ views and interests," said House Minority Leader Nancy Pelosi. "As he works to address his health, our thoughts and prayers are with him, his wife, Sandi, his children as well as his parents."

Jacksons’ resignation means the 112th Congress that ends next month has 241 Republicans and 192 Democrats. The other vacancy is Rep. Dennis Cardoza, a California Democrat who resigned in August.

Jackson came to Congress in a special election in December 1995 to succeed another embattled Illinois Democratic congressman, Rep. Mel Reynolds, who went to jail for sex misconduct chargers and resigned several months before the special election.

Jackson defeated five Democrats and four Republicans in the primary and then won the special election.

"The constituents of the second district deserve a full-time legislator in Washington, something I cannot be for the foreseeable future," Jackson also said in his resignation letter. "My health issues and treatment regimen have become incompatible with service in the House of Representatives. Therefore, it is with great regret that I hereby resign ... effective today in order to focus on restoring my health."