BBC boss Greg Dyke has blamed John Cleese for declining sitcom standards.
He said that writers of hit TV comedies prefer to write only a couple of series before moving on to something new - a trend started with Fawlty Towers.
But it means the corporation is Losing It best shows too quickly.
Both Victoria Wood and Caroline Aherne stopped writing dinnerladies and The Royle Family respectively, despite their success, because they felt the shows had run their course after only a limited number of episodes
He added that Only Fools and Horses had run for 15 years to achieve its place in the nation's hearts.
He also said that the Del Boy comedy "is still likely to come back".
Dyke was being quizzed about the state of sitcoms at the presentation of the BBC's annual report this lunchtime.
At the event, Alan Yentob, the BBC's director of drama and entertainment, said the state of British sitcom was not as bad as the press had reported.
"It's never easy finding hit sitcoms," he said. "You need to persist.
"You have to be patient, and it often requires an act of faith to make it happen.
He said that a show like One Foot In The Grave would not look good on paper, as nobody would apparently be interested in a show about a whinging pensioner, "yet it did connect with all ages.
And Gimme Gimme Gimme was an unexpected hit, he suggested.
"We all held our breath and put in on BBC2," he said. "Those who have seen it know what I mean. Those who haven't should probably keep away."
But Yentob added that there were other "signs of life" in sitcoms, citing Happiness, Coupling, Two Pints of Lager and a Packet Of Crisps as examples of promising series.