"I regret the harm that my actions have caused my family, my staff and my constituents," Lee posted in a surprise announcement Wednesday night on his congressional website. "I deeply and sincerely apologize to them all. I have made profound mistakes and I promise to work as hard as I can to seek their forgiveness."
A woman described as a 34-year-old Maryland resident and government employee provided the Gawker website with e-mails she said were an exchange between her and Lee in response to an ad she placed last month in the "Women Seeking Men" section of Craigslist.
Gawker reported Wednesday that Lee, 46, identified himself as a divorced 39-year-old lobbyist and sent a photo of himself posing in front of a mirror.
In one e-mail, Gawker said, Lee described himself as "a very fit fun classy guy" and promised "not to disappoint." He and the woman exchanged a few more messages, talking about recent dates and sharing biographical details.
In one, the site shows, the woman asks Lee if he's divorced.
Yes, he replied, one minute later.
The woman told Gawker she eventually broke off the contact with Lee after becoming suspicious that he had misrepresented himself.
Lee, a two-term Republican with a young son, said in an e-mailed statement that his resignation was effective immediately. The statement offered no confirmation or details of a Craigslist post.
On Thursday, House Speaker John Boehner said Lee's rapid resignation was his decision alone.
"I think he made the right decision for himself and for his family," the Ohio Republican told reporters in Washington.
Boehner said he became aware of the picture sometime the previous afternoon and of the resignation just after 6 p.m. Boehner refused to say, however, whether he spoke to Lee or urged him to resign.
Lee, who won his seat in 2008, cultivated a family-values voting record in the House, earning an 88 percent approval rating from the American Conservative Union for his 2010 votes. He voted in favor of a ban on federal funding of abortion in the health care overhaul, in line with the group's position on the proposed ban, which was defeated in the House. He also voted against the repeal of the military's policy prohibiting service by openly gay men and women.
He served on the House Ways and Means Committee and was active on economic revitalization issues. He has a business background stemming from his family's manufacturing enterprises.
Lee said the challenges faced in western New York, where he serves the 26th Congressional District, and across the country are "too serious for me to allow this distraction to continue, so I am announcing that I have resigned my seat in Congress effective immediately."
Lee's resignation comes almost a year after Democrat Eric Massa resigned his seat in western New York's 29th Congressional District amid an investigation into whether he sexually harassed male staffers. Massa gave contradictory explanations for his behavior, acknowledging he groped and tickled a staffer in a nonsexual way and wrestled with others at his 50th birthday party, but then denying any groping occurred.
His resignation comes as the people of Clarence, the upscale Buffalo suburb where he lives, are about to mark the two-year anniversary of the deadly crash of Continental Connection Flight 3407 onto a house.
He has been a visible supporter of victims' families and the aviation safety reforms they've sought since the Feb. 12, 2009, crash. He appeared with several crash victims' families at a news conference in Washington on Tuesday.
Scott Maurer, whose daughter was one of the 50 people killed in the 2009 crash, said Lee was "a hard worker and a good guy" who "loved and cared for" his community.
"He was 100 percent professional with us, 100 percent supportive and seriously was considerate and concerned with aviation safety, so to hear that he's resigned is a huge loss of support for our efforts," Maurer said.
The congressman hadn't submitted his resignation to the New York secretary of state Wednesday evening, said a state official who wasn't authorized to speak on the issue and spoke on the condition of anonymity. After Lee submits his resignation, Gov. Andrew Cuomo could schedule a special election.
Cuomo, a Democrat, had no comment Wednesday night.
Associated Press writers Andrew Miga and Laurie Kellman in Washington and Michael Gormley in Albany contributed to this report.