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.

The advisory role with Mr. Trump had raised concern because Mr. Icahn had taken a strong view on regulatory issues that could have benefited some of his investments.

One of his investment firms, Icahn Enterprises, owns a big stake in an oil refinery business called CVR Energy. And Mr. Icahn has been vocal about environmental regulations. In a front-page article in March, The New York Times detailed how Mr. Icahn had pressed for a change in a requirement that refiners be held responsible for ensuring that corn-based ethanol is mixed into gasoline.

“Following his apparent failure to win a regulatory change on ethanol rules that would benefit his personal empire, Carl Icahn is taking his ball and going home,” Robert Weissman, the president of the nonprofit advocacy group Public Citizen, said Friday....Read more at NY Times