Franks, an eight-term lawmaker from Arizona, had announced a day earlier that he would be stepping down from the House of Representatives on January 31.
In that statement, he acknowledged that he had discussed surrogacy with two female staffers, but he did not provide details about the discussions.
In a new statement Friday, he said he was vacating his seat in Congress immediately, US media reported.
Politico, citing congressional sources with knowledge of the complaint against Franks, reported explosive accusations from two female subordinates.
Franks approached the staffers about acting as a potential surrogate for him and his wife -- Franks says he and his wife have struggled with fertility for years.
The women, according to the sources, were concerned that the congressman was asking to have sex with them, and that it was unclear whether he was asking about the prospect of impregnating them through sexual intercourse or in vitro fertilization, Politico reported.
The news outlet said Franks denies all of the accusations, according to a spokesman.
In his initial statement, Franks, a conservative Republican opposed to abortion rights, said he "absolutely never physically intimidated, coerced, or had, or attempted to have, any sexual contact with any member of my congressional staff."
House Speaker Paul Ryan was told of the misconduct allegations on November 29, his office said, adding that Ryan believed the accusations were "credible" and that he told Franks he should resign.
Ryan's office declined to comment on the latest accusations, or Franks's immediate resignation.
The scandal comes as sexual harassment allegations have engulfed Capitol Hill.
Franks is the third member of Congress this week -- following House Democrat John Conyers and Senate Democrat Al Franken -- to announce his resignation in the wake of sexual misconduct accusations.
