So how many Deep Throats need to sing?

Deep Throat is the most famous anonymous journalistic source of them all. His identity still remains a secret. But the young Washington Post journalists investigating the Watergate scandal in the 1970s always backed up their source's leaks with information from other insiders in the Nixon administration.

If Bob Woodward and Carl Bernstein could use it to bring down the President, it is no surprise that some journalists see the "two source rule" as unbreakable. "We would usually require that [a story] comes from more than one source," says Len Downie, editor of The Washington Post, who worked at the newspaper at the time of the scandal. "It is a very, very rare occasion that we use a single source. There was never a story based just on what [Deep Throat] said."

This unwritten rule of journalism is now the subject of a vitriolic row between the BBC and the Government, or more precisely, between Alastair Campbell, Tony Blair's spin doctor, and Andrew Gilligan, the defence correspondent of the BBC.

No 10's complaint is that Mr Gilligan ran a story on Radio 4's respected Today programme, based on just one anonymous "British official". The allegation was that No 10 "sexed up" information from the secret services in a dossier, intent on convincing a sceptical British public to go to war. Mr Campbell has loudly declared the allegation to be a "lie" and has demanded an apology, but the BBC's director general, Greg Dyke, has stood by the story. The BBC says that in sensitive issues of intelligence and defence, to use one, credible source as the basis of a story is justified.

But Mr Campbell's argument is that the story has been denied by Tony Blair, Jack Straw, Sir Richard Dearlove (head of the Secret Intelligence Service) and others. "When the BBC defence correspondent, on the basis of a single, anonymous source, continues to say that it is true, then I think something has gone very wrong with BBC journalism," he told the Foreign Affairs Select Committee late last month.

However, the use of a single anonymous source is by no means uncommon. In a survey of wire agencies, newspapers and broadcasters, The Independent on Sunday found that no single company would kill an important story because it had only one, unnamed source.

"The number of sources needed is treated on a story-by-story basis," said a spokesperson for ITN.

Sky News has a similar policy, although its reporters tend to work on fast-moving stories like the murder of the television presenter Jill Dando rather than indepth political investigations. "Our crime correspondent, Martin Brunt, heard from an 'impeccable' source that Jill Dando had been attacked, and Sky News broke the news," says John Ryley, an executive editor at the channel. "Later, the same source told Brunt she was dead. This time, Brunt double sourced the story - it took him only a few minutes - then broke the news of Jill Dando's murder ahead of our rivals."

American companies are more reluctant to use a single source for a story. Bloomberg, the financial news agency, said a story based on a single, anonymous source would require approval from an executive editor. The New York Times describes using anonym-ity as a "last resort", although it might use a single source on a "case-by-case" basis.

In the UK press, the policies are pretty closely aligned with the BBC's. "We treat every case strictly on merit," said Colin Randall, deputy news editor of The Daily Telegraph. "[We] always apply common sense and recent history, our dealing with the correspondent and the quality of his or her sources in the past, and we are guided by his or her account of the quality of the source."

"It depends on who the source is, and who the source is talking to," says the editor of The Independent on Sunday, Tristan Davies. "Information provided by a source needs to be corroborated, but if the provenance of the material is unquestionable, then that corroboration need not be a second source providing or confirming the information provided by the first. Clear as mud, isn't it? Which is how it should be: 'sources close to' and 'friends of' should be confident that off-the-record briefings and on-the-record unattributable comments will be treated in the strictest confidence. The point is not the number of sources, but the quality of the material and of the provider."

Stephen Jukes, Reuters' head of global news, says that a single source could be used "only if we were convinced the source was credible and knew exactly what he or she was talking about". When Margaret Thatcher resigned as Prime Minister, Mr Jukes approved a story that came from one "well-informed" source who was "right up there".

However, when a source is anonymous, the reader or listener must rely on the journalist's judgement of the source's credibility. Mr Gilligan has said his source was "one of the senior officials in charge of drawing up the dossier, a source of longstanding, well-known to me, closely connected with the question of Iraq's weapons of mass destruction, easily sufficiently senior and credible to be worth reporting".

Whether his story was right or wrong, journalists still stand by the principle that they should be able to use single sources if necessary. "It is essential for democracy to get information ... sometimes involving anonymous sourcing and, in an extreme situation, a single source, if you feel good about their credibility," notes The Washington Post's editor, Mr Downie.

Without knowing more details about Mr Gilligan's source, most editors say they could not decide whether they would have run the BBC's story. If he or she remains as veiled as the original Deep Throat, we may never know whether the BBC was brave - or reckless.