In the face of protests from historians and Jewish groups, the US broadcaster "C-Span" backed down from plans to air a lengthy speech from David Irving, an author whom critics and a British court have labeled a holocaust denier. Instead, the network's popular "Book TV" program included only two brief Irving video clips. In 2000, the controversial historian lost a highly publicized libel case against an American professor, Deborah Lipstadt. The professor had accused Irving of being an anti-Semite and of grossly distorting evidence in order to prove that the Holocaust was largely a myth. "C-Span" had initially planned to carry a talk delivered by Lipstadt last month to the Harvard Hillel student organization. However, Lipstadt withdrew permission for the network to tape her remarks after she learned that the cable outlet planned to pair her comments with a lecture Irving delivered recently at a dinner in Atlanta. She said the consecutive appearances would violate her longstanding policy of not engaging in debate with Holocaust deniers. The compromise program aired yesterday left both Lipstadt and Irving sharply critical of the network.