Carl C. Icahn, the billionaire investor, is no longer advising President Trump on regulatory matters.

In his first post on Twitter in three months, Mr. Icahn announced on Friday that “with President Trump’s blessing, I ceased to act as a special adviser to the president on issues relating to regulatory reform.”

The post linked to a letter on his website, in which he stated that he “never had a formal position” with the Trump administration.

Mr. Icahn announced his resignation a few hours before The New Yorker published an online article about the conflicts created by his advisory role. The writer, Patrick Radden Keefe, later said on Twitter that the White House told him Mr. Icahn had been fired as an adviser on Monday.