NEW YORK (NYTIMES) - Nate Diaz, a popular and outspoken star of the Ultimate Fighting Championship (UFC), said that he would not fight next week at New York's Madison Square Garden, as planned, because he had tested positive for a banned substance.

In a text message, he said that he was unsure what the banned substance is.

But on Thursday afternoon, 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 text message that he did not know the substance that caused a positive result.

"They won't tell me, they're being shady, I got nothing to b shady about," he wrote. He did not immediately respond to follow-up questions.

He was scheduled to fight Jorge Masvidal on Nov 2 in a welterweight fight to headline a card billed as UFC 244.

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 Diaz 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 Diaz stopped fighting for the better part of three years.

He returned in August to great fanfare and beat Anthony Pettis. His fight with Masvidal in New York was seen as a possible avenue back to a third fight against McGregor, who is mired in sexual-assault allegations but who announced a January return to the Octagon on Thursday.

"Until UFC, Usada (United States Anti-Doping Agency), or whoever is (expletive) with me Fixes it, I won't be competing," Diaz told his followers in a tweet filled with misspellings. "Im not gonna play their game and try to hide it or keep quite(sic), as they suggested. I'm not gonna have my name tainted as a cheater."

He said other fighters 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."