Rep. Chris Collins, R-N.Y., leaves the House Republican Conference meeting in the Capitol on Wednesday, June 20, 2018.

Rep. Chris Collins, R-N.Y., submitted his resignation from Congress on the eve of his scheduled change-of-plea hearing in an ongoing federal insider trading case, a spokesman for House Speaker Nancy Pelosi told CNBC on Monday.

Drew Hammill, a spokesman for Pelosi, D-Calif., said that the speaker's office is "in receipt of a letter of resignation" that "will be laid down on the House Floor tomorrow" during a 9 a.m. pro forma session in the chamber.

"Resignation will be effective at that time," Hammill said.

A spokeswoman for Collins' congressional office did not respond to CNBC's requests for comment.

Collins' resignation letter to Pelosi, provided to CNBC by a person familiar with the matter, reads: "I hereby submit my resignation effective immediately, September 30, 2019, as United States Representative of the 27th District of New York. Attached is the letter I submitted to Governor Andrew Cuomo of New York."

Collins' decision to vacate his seat in Congress came just hours after court documents revealed that the House Republican — who was the first member to support President Donald Trump's 2016 campaign — was set to change his plea in the insider trading case that has dogged him for over a year.

Both actions show a remarkable pivot from just a few weeks earlier, when Collins appeared to be gearing up for fights in both the legal and political arenas.

Collins, 69, pleaded not guilty after he was arrested in August 2018 on charges that he shared private information related to Australian biotech company Innate Immunotherapeutics, where he served as a board member.

He is accused of calling his son, Cameron Collins, in a panic from the White House lawn to share information about a failed drug trial. He and others then allegedly sold their stock based on that tip before the trial results became public.

Cameron, and the father of his fiancee, Stephen Zarsky, are also charged in the case.