A US television comedian and actor who claimed he narrowly escaped the World Trade Center on 9/11 has admitted fabricating the entire story.

Steve Rannazzisi, who stars in the FX fantasy football sitcom The League, said his lie about fleeing the South Tower before the second plane hit was "inexcusable".

The 37-year-old stand-up, who has a one-hour special due to air this Saturday on Comedy Central, confessed after being challenged about inconsistencies in his story.

Rannazzisi claimed to have been working at Merrill Lynch on the 54th floor of the doomed skyscraper, but the bank says it had no offices in the building, nor any record of employing him.

He has told the story a number of times, claiming his bogus brush with death inspired him to quit his desk job and chase his dream of being an entertainer.

"I was there and then the first tower got hit and we were like jostled all over the place," he told an interviewer in 2009.

"I still have dreams of like, you know, those falling dreams."

In a 2011 interview, Rannazzisi said he had received a good severance package from Merrill Lynch.

"I've spoken about it (9/11) before," he said. "I just don't ever want to feel like, anyone, I am cashing in or anything like that."

He said on Twitter on Wednesday that he began telling people he was in the World Trade Center on 9/11 shortly after moving with his wife from New York to Los Angeles, following the terrorist attacks.

"It wasn't true," he tweeted. "I was in Manhattan but working in a building in midtown and I was not at the Trade Center on that day".

"I don’t know why I said this. This was inexcusable. I am truly, truly sorry.

"For many years, more than anything, I have wished that, with silence, I could somehow erase a story told by an immature young man."

"It only made me more ashamed," the father-of-three continued. "How could I tell my children to be honest when I hadn't come clean about this?

"it is to the victims of 9/11 and to the people that love them - and the people that love me - that I ask for forgiveness.

"It was profoundly disrespectful to those who perished and those who lost loved ones.

"The stupidity and guilt I have felt for many years has not abated. It was an early taste of having a public persona, and I made a terrible mistake."

Rannazzisi is also the face of an ad campaign by sports bar franchise Buffalo Wild Wings.

The company said on Tuesday night in a statement to the New York Times: "We are currently re-evaluating our relationship with Steve pending a review of all the facts."