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

It was never clear exactly what Mr. Icahn’s role was — he did meet at one point with Jay Clayton before Mr. Clayton was approved by the Senate as chairman of the Securities and Exchange Commission. It is also believed that Mr. Icahn had only a few conversations with Mr. Trump since the president’s inauguration.

Mr. Icahn has been especially vocal about environmental regulations and one of his investment firms, Icahn Enterprises, owns a big stake in an oil refinery business called CVR. Earlier this year, several Democratic senators demanded that Mr. Icahn step away from his advisory role because of the potential conflict of interest.

Reached at his home, Mr. Icahn declined to elaborate beyond his letter and Twitter posts on the matter.

“I am not going to say anything more,” Mr. Icahn said. “I really can’t talk about this.”

Mr. Icahn was bullish on Mr. Trump’s election. The morning after Mr. Trump won the election, Mr. Icahn said that he made $1 billion worth of bets in the stock market.

In his letter on Friday, Mr. Icahn responded to the criticism about his role advising Mr. Trump by saying that he “never had access to nonpublic information or profited from my position.” He added that he did not believe his role had created a conflict of interest.