By Lynette Rice
Updated January 04, 2008 at 05:45 AM EST

Though his Tonight Show monologue was irreverent, topical, and clearly strong enough to help him win the night in viewers (7.19 million versus The Late Show With David Letterman‘s 5.5 million), Jay Leno‘s jokes on his first show back didn’t sit too well with the Writers Guild of America. Today, the union issued a statement saying that it talked with Leno to “clarify that writing for The Tonight Show constitutes a violation of the Guild’s strike rules.”

Leno, who is a member of the WGA, admitted on camera last night that he wrote his own jokes. (Sample: He compared the amount of money lost in the writers strike to what Paul McCartney could lose in a divorce. He also quipped about how the public relies on him to get the latest news about Britney Spears.) What’s more, it was clear during the telecast that Leno was reading from cue cards. The WGA has instituted a “pencils down means pencils down” policy for all of its members during the strike, now in its third month.

NBC issued its own statement disagreeing with the WGA’s assessment of Leno’s performance last night. “The WGA agreement permits Jay Leno to write his own monologue for The Tonight Show. The WGA is not permitted to implement rules that conflict with the terms of the collective bargaining agreement between the studio and the WGA.”

Leno was one of the first celebrities to show solidarity for the strike that began Nov. 5 by joining the writers on the picket line. He said in his monologue last night that he needed to go back on the air “because we have essentially 19 [writers] putting 160 [crew] people out of work.”