Williams was suspended last year for violating the substance abuse policy and was eligible to be reinstated this month, but the league's medical advisers who oversee Williams' personal rehabilitation program recently notified commissioner Roger Goodell of the positive test, sources said.
Williams, through his agent, Leigh Steinberg, issued a statement to The Associated Press on Friday night:
"Due to the recent reports about me failing a drug test, I feel it is appropriate for me to issue this statement. Last month, following a psychological evaluation requested by the NFL, we -- the psychiatrist and I -- came to the realization that there were a few things I needed to iron out about myself in order to make my return to the NFL as successful as possible.
"I am an honest, God-fearing man who is intensely dedicated to being the best person I can be on and off the football field. There is no need to smear my name or to defame my character for the sake of news. When the time is right, God willing, I will be back on the field scoring touchdowns for whatever team is fortunate enough to believe in me."
The doctors also recommended that Williams not be allowed to apply for reinstatement until September, sources said. The commissioner usually adheres to the advice of the medical advisers under the substance abuse policy.
"Falling off the wagon is part of rehab," a source said. "Based on the medical evidence in Ricky's case, the doctors say it's too early to come back. He had the positive test last month. Remember, he's been diagnosed with social anxiety disorder -- that's a real disease and a good percentage of those folks self-medicate with substances like marijuana, often at the moment they are about to have a high level of social interaction."
League spokesman Greg Aiello said the NFL would have no comment, as did Dolphins spokesman Harvey Greene.
David Cornwell, an attorney who has represented Williams during his substance abuse cases, said he would honor the confidentiality policies in effect with another no comment.
Williams, who turns 30 on May 21, has played only 12 games since 2003. Before his latest setback, new Miami coach Cam Cameron repeatedly declined to say whether he would welcome Williams back.
The NFL suspended Williams in April 2006 after he violated the league's drug policy for the fourth time. That failed test apparently involved a substance other than marijuana and may have been related to his interest in holistic medicine.
Williams' previous positive drug tests were for marijuana, which he acknowledged using.
Last year Williams played in the Canadian Football League, then taught yoga in California. Steinberg recently quoted Williams as saying his interest in Eastern philosophy had overcome his desire for mind-altering substances.
Williams still owes the Dolphins $8.6 million for breaching his contract when he sat out the 2004 season, but they've given no signal they want him back.
Williams won the 1998 Heisman Trophy at Texas and the 2002 NFL rushing title with the Dolphins. Last season with the Toronto Argonauts of the CFL, he rushed for 526 yards in 109 carries but missed two months with a broken arm.
"I appreciate all the support I have received from my fans," Williams added in his statement, "and I assure all others that I am strong, clean, and happily preparing myself for a triumphant return to the NFL."
Chris Mortensen covers the NFL for ESPN. Information from The Associated Press was used in this report.