NEW YORK • Nate Diaz, a popular and outspoken star of the Ultimate Fighting Championship, said that he would not fight at the UFC 244 event as planned because he had tested positive for a banned substance.

On Thursday, in a tweet that was light on details but heavy on profanity, he told his 1.3 million followers that he would not be fighting "because they say I tested with elevated levels that they say might be from some tainted supplements".

"I call false on that," he added.

Asked for clarification, he said in a subsequent message that he did not know what the substance was.

"They won't tell me, they're being shady, I got nothing to b shady about," he wrote.

In his first tweet, he said others keep quiet until after they fight in order to get a payday.

"That's cheating," he wrote. "So fight game I'll see you when I see you."

He was scheduled to fight Jorge Masvidal next Saturday in the main event, a welterweight fight at New York's Madison Square Garden.

Asked about Diaz's disclosure, a UFC spokesman said that she is "looking into this" and had no other immediate comment.

The UFC is accustomed to late changes to its fight cards, but any cancellation involving the popular fighter could be very costly, at the box office and in pay-per-view receipts, because of his popularity.

Diaz, a pot-smoking, unfiltered anti-hero from Stockton, California, was a journeyman fighter who beat Conor McGregor in 2016 as a last-minute replacement. The Irishman avenged the defeat five months later, and the American stopped fighting for the better part of three years.

He returned in August to great fanfare and beat Anthony Pettis. His fight with Masvidal was seen as a possible avenue back to a third fight against McGregor, who announced a return to the octagon in Las Vegas in January on Thursday.

NYTIMES