Ex-presidential candidate Al Gore's wife Tipper was a tireless campaigner

"Understandably the public is outraged by the increasingly coarse content aired on radio and television at all hours of the day," FCC commissioner Susan Ness told the newspaper Daily Variety.

The order, issued on 6 April, refers to several examples of early morning radio banter and songs in pointing out what might be offensive to the listening public.

The FCC says it has received a "flood" of letters and e-mails reflecting the public's "anger" at such content.

Smut

The commission does not monitor the airwaves, but waits for the public to bring its complaints to them.

If a broadcaster is cited for airing indecent content, the FCC can revoke a station's licence, impose a fine or issue a warning.

But the commission must be careful not to infringe US rules on free-speech.

"Despite the onslaught of on-air smut, the commission necessarily walks a delicate line when addressing content issues and must be careful not to tread on the First Amendment."

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The First Amendment to the US constitution enshrines freedom of speech - something DJs like Stern have been exploiting.

The commission was first directed by a court to define what constitutes "offensive" output in 1994.

It has taken until now to do so.

Under the new ruling broadcasters must refrain from explicit and graphic chat, not titillate viewers or listeners and must not plan to shock with their output.

The ban runs from 0600 to 2200 daily.

Foul-mouth

Some campaigners against indecency on-air have already complained that the FCC has not gone far enough.

The US has long struggled with how to combine the right to freedom of speech without offending listeners or viewers.

Al Gore's wife Tipper was a high profile campaigner for an end to violent or explicit lyrics on songs.

She succeeded in getting warning stickers put on CD's with adult content.

Most new US television sets are fitted with a V-chip, which allows parents to monitor the television their children watch.

Shock jocks have not taken off in the same way in the UK. The genre's most famous British exponent was Caesar The Geezer, who appeared on Kiss 100 in London and national station Talk Radio.

Last year the Radio Authority fined London-based Xfm and Hallam FM in Sheffield a record �50,000 each for offensive broadcasts, after they broadcast material which condoned rape and included offensive language, bestiality and descriptions of paedophilia.

Xfm, owned by the Capital Radio group, was warned at the time that it risked losing its broadcasting licence.