Journalists have a moral duty to protect the confidentiality of their sources, even at the cost of going to prison themselves.
As a result of one young journalist, Bill Goodwin, being prepared to take that risk, the European Court of Human Rights ruled in his case against the UK that all journalists had a presumptive right to resist legal demands to identify those who privately pass on information: "Protection of journalistic sources is one of the basic conditions for press freedom ... without such protection, sources may be deterred from assisting the press in informing the public on matters of public interest.
"As a result the vital public watchdog role of the press may be undermined and the ability of the press to provide accurate and reliable information may be adversely affected."
The cultivation of sources is thus essential, for journalists and for the public they serve.
It is a basic tool of their trade, the means by which newsworthy information is extracted from powerful corporations and government departments which want to keep wrongdoing secret or give it a particular 'spin'.
Without the ability of journalists to promise anonymity to sources who fear reprisals, and to keep that solemn promise, there would be a lot less news and what there is would be less reliable.
Thanks to the rule in Goodwin's case, the media in Europe have much better source protection than in the US, where the Obama Administration, for all its pretended support for press freedom, has brought more prosecutions against current or former government officials for providing classified information to the media than any previous administration.
But even journalists in Europe must be ultra-careful: if, for example, they refer to sources on notes they tap into their office computer, they may find that their corporation (which owns the computer) can hand it over to the police.
This was actually done by Rupert Murdoch, when in the interests of his US corporation, he authorised the handing over to police of all computer records for his tabloid newspapers, thus enabling the prosecution of a number of sources in the army and the police that his journalists had cultivated for public interest stories.
Murdoch's malign and self-interested behaviour put his journalists in breach of their ethical duty to their sources, but it does serve as a reminder that this duty is not absolute.
There are circumstances where a journalist or editor is entitled to breach that duty - e.g. where the source has maliciously fed them a false story, or encouraged them to publish information from which they have made a secret profit.
If source revelation would make the difference to guilt or acquittal of a defendant on a serious criminal charge, then there will be a duty to disclose.
Sources should normally be paid no more than their expenses, although there may be cases where the public interest would justify more substantial payments, e.g. for legal advice or for additional consultation, or where a story of great public interest cannot otherwise be obtained.
Some sources give themselves away (the case with Bradley Manning, the alleged 'leaker' to Wikileaks) but for most stories hailed as important examples of investigative journalism, there will be nervous people fearing reprisals if their cover should be blown.
Most would be sacked from their jobs, whilst others might be jailed or even killed. They are the real heroes of investigative reporting, and they must not be abandoned.
Geoffrey Robertson QC is a Human Rights lawyer based in the United Kingdom.