A warning to all network marketing gurus: You might want to triple-check before airing any more outlandish ratings claims, because the primetime promo police are watching.

Nielsen Media Research plans to set up a Web site that will serve as a clearinghouse to back broadcast and cable on-air ratings claims (“We’re No. 1!” or “We’ve got the top-rated new series!”) with cold, hard Nielsen data.

“Supporting info will be available on the Web site, so that the networks can check out how their rivals are supporting their ratings claims,” said Nielsen spokesman Jack Loftus.

As fewer ratings points than ever before separate first from worst, the networks have gotten craftier with their numbers. Webheads will traditionally contact Nielsen if they find a rival’s claim questionable; those disputes have become more common as everyone claims to be No. 1 in something.

“With more networks out there, everyone is making claims about everything,” said George Schweitzer, CBS exec VP for marketing and communications. “The world of claims advertising is getting very crowded. But there’s nothing wrong with trying to tout your performance. We’ve always vetted our claims.”

NBC research and media development prexy Alan Wurtzel said he found it ironic that the webs’ broadcast standards departments fully check out any claims made by their advertisers– but let their own promos go largely unregulated.

“Everybody should go out and tell the best story they can about their efforts,” he said. “Let’s just do it accurately.”