NEW YORK (CNN/Money) - After more than a decade as the dominant broadcast network, NBC has seen its viewership tumble, enduring double-digit ratings declines in total viewers and viewers aged 18 to 49, according to a newspaper report Wednesday.

NBC is owned by General Electric. (Research)

For the first time since 1987, NBC is losing viewers for its Thursday night line-up, the night that draws the most advertising dollars and has been the network's most important popular night of television, the Wall Street Journal reported.

"You never want to draw a firm conclusion from a few weeks, but NBC's chances of regaining the lead appear to be limited," Lowell Singer, an analyst at SG Cowen & Co. told the Journal.

But the station's declines have been steeper than what the industry considers acceptable, down 12 percent in overall viewers and 16 percent in the 18-to-49 age category, said the newspaper, adding that executives at four big ad-buying companies say NBC may fail to meet ad guarantees and could be forced to give away time to advertisers.

Just six months ago, chairman of NBC Universal Television Group Jeff Zucker talked up the company's hold on the No. 1 spot, raking in $2.9 billion in advertising commitments for the 2004-2005 season, said the report, more than any other network.