NEW YORK — Peter Jennings, the chief ABC News anchorman for more than 20 years, has been diagnosed with lung cancer and will begin outpatient treatment next week, the network said Tuesday.

Jennings, 66, told ABC News staff of his diagnosis Thursday morning and said he will continue to anchor the broadcast when he feels up to it as he begins chemotherapy.

He last anchored World News Tonight on Friday and was too ill to work Saturday during the network's special report on Pope John Paul's death.

"There will be good days and bad, which means some days I may be cranky and some days really cranky,'' he told ABC News employees in an e-mail.

Jennings, who was born in Toronto in 1938 and raised in Ottawa, began his broadcasting career in Canada. At the age of nine, he hosted his own weekly children's show on CBC Radio.

In 1962 he became co-anchor of the CTV national newscast and two years later moved to New York to join ABC-TV News. Jennings became a U.S. citizen in 2003 but still retained his Canadian citizenship.

Charles Gibson, who's in Rome for the Pope's funeral, and Elizabeth Vargas will be Jennings' primary substitutes on the evening news.

Jennings is the last of the anchor troika that dominated broadcast U.S. network news divisions over the last two decades. NBC's Tom Brokaw stepped down last year and CBS's Dan Rather left the evening news last month.

A former foreign correspondent based in Rome and London, the urbane Jennings and World News Tonight dominated the ratings during the late 1980s and early 1990s, before being surpassed by Brokaw. His broadcast is now a close second to NBC's Nightly News with Brian Williams.

He has been feeling ill for the past several months. Following the tsunami in December, he did not travel to the stricken region, with ABC explaining at the time that he had an upper respiratory infection and was under doctor's orders not to travel. He did go to Iraq in January for the elections.

Jennings is a former smoker who gave up the habit several years ago, the network said.

He anchored ABC's evening newscast for two years in the 1960s, then joined a multi-anchor format in 1978. ABC abandoned the approach in 1983 after Frank Reynolds died of cancer, and Jennings has been the sole anchor since then.