Out of 21 episodes of the cult Sixties comedy series 16 were wiped by the BBC between 1970 and 1974. The five that remain were filmed illegally from a TV screen at the time of broadcast by the series producer who was scared of the tapes being lost.
The story emerges from a biography of Peter Cook (right) published today by comedy producer Harry Thompson.
Mr Thompson describes the wiping of the tapes as an act of cultural vandalism and has tried to find out who in the BBC's senior management ordered the wiping.
"Jimmy Gilbert, head of comedy at the time said there was no opposition to the order," said Mr Thompson yesterday.
"People didn't question it. Comedy wasn't seen as a cultural artefact to be saved.
"Instead the main priority was to keep news programmes. Every single dumb local news item had to be kept."
Mr Thompson met Peter Cook after working in the BBC's archives in the Eighties. He discovered the few remaining episodes and copied them onto a VHS tape. This he presented to Mr Cook and saved a copy for himself.
The BBC has subsequently used the few tapes left in compilation to show a Best of Not Only ... But Also. "That's why the so-called classic scenes from the series are the only ones you ever see," said Mr Thompson, "It's the only ones they've got."
Not Only ... But Also grew out of Cook and Moore's collaboration in the hit satire Beyond The Fringe. The BBC's policy on keeping tapes also meant that the black and white episodes of Steptoe and Son shown by the BBC two years ago were from tapes made illegally by a fan in Australia. The originals had been destroyed.
A BBC Resources spokesman said "If anyone has any tapes from that time we would love to hear from them and take them back into the archive."