Rep. Chris Collins, R-N.Y., has sent a letter to House Speaker Nancy Pelosi’s office stating that he is resigning from his House seat, a source familiar with the matter tells NBC News.

Collins' resignation comes ahead of his expected guilty plea Tuesday to charges relating to insider trading, according to documents filed in federal court Monday. His resignation will become effective once his letter is read on the House floor during Tuesday’s pro-forma session.

Collins, the first member of Congress to announce his support for Donald Trump's presidential bid, is scheduled to appear for a "change of plea" hearing in a Manhattan courtroom at 3 p.m. Tuesday. He pleaded not guilty to insider trading and several other charges when he was first indicted in 2018. Experts say the hearing means he is likely changing his plea to guilty.

Collins' lawyer did not immediately respond to a request for comment.

Collins, 68, who endorsed Trump for president in February 2016, was charged with insider trading in August 2018 along with his son, Cameron, and Stephen Zarsky, the father of Cameron's fiancée, according to the indictment. The men are also charged with lying to the FBI in interviews to cover up the alleged scheme to profit off insider knowledge about a company called Innate Immunotherapeutics.

In a news conference following his indictment, Collins said, "I believe I acted properly and within the laws at all times."

"Throughout my tenure in Congress, I have followed all rules and ethical guidelines," he added.

Cameron Collins and Zarsky will also have change of plea hearings Tuesday, and are also likely to change pleas of not guilty to guilty.

A spokesman for the U.S. Attorney's Office in Manhattan had no comment on the court filing.

According to a grand jury indictment, in June 2017 Collins passed along material that was nonpublic regarding the results of a drug trial — meant to treat an advanced form of multiple sclerosis — so his son "could use that information to make timely trades in Innate stock and tip others."

The drug trial failed — and Innate's stock would later tumble by 92 percent.

Collins was at a congressional picnic at the White House on June 22 when Innate's chief executive sent an email to the board of directors reporting that the trial was a bust, according to the indictment.

"I have bad news to report," the email began.

Collins quickly replied: "Wow. Makes no sense. How are these results even possible???"

Then, within six minutes came a flurry of phone calls to reach his son, who owned more than 2 percent of Innate stock, prosecutors said. He finally reached Cameron Collins, who allegedly passed the information from his father to Zarsky and other unnamed co-conspirators, who then engaged in "timely trades" of the stock.

Cameron Collins unloaded his Innate shares, as did his fiancée and Zarsky, in the days that followed. Zarsky's wife and a friend also benefited from the move, prosecutors said.

On June 26, news of the failed drug trial was made public and the stock took a nosedive. The defendants managed to avoid more than $768,000 in losses, prosecutors allege.