The Guardian, Financial Times, Independent and Times newspapers and the Reuters news agency have won their case at the European Court of Human Rights.
The publishers ran stories in 2001 claiming that Belgian brewer Interbev was mulling a bid for South African Breweries, news that caused fluctuations in the share prices of both groups.
The stories were based on leaked documents from a presentation by Interbev's financial advisers Goldman Sachs. The brewer hired corporate risk firm Kroll to identify the leaker and that led to a demand for copies of the documents.
The publishers refused and defended themselves at the High Court and the Court of Appeal when both of those courts ordered the release of the documents. The publishers refused, arguing that the protection of sources was vital to the continuing practice of their journalism.
When the House of Lords refused them permission to appeal any further they took their case to the European Court of Human Rights, which rules on countries' compliance with the European Convention on Human Rights (ECHR).
They argued there that any court order which forced them to give up the documents would be an unlawful interference with their right to freedom of expression as guaranteed by Article 10 of the ECHR.
That clause says: "Everyone has the right to freedom of expression. This right shall include freedom to hold opinions and to receive and impart information and ideas without interference by public authority and regardless of frontiers ... The exercise of these freedoms, since it carries with it duties and responsibilities, may be subject to such formalities, conditions, restrictions or penalties as are prescribed by law and are necessary in a democratic society ... for the prevention of disorder or crime, ... for the protection of the reputation or rights of others, for preventing the disclosure of information received in confidence".
"Protection of journalistic sources is one of the basic conditions for press freedom," said the European Court's ruling. "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 reporting may be adversely affected."
"Having regard to the importance of the protection of journalistic sources for press freedom in a democratic society and the potentially chilling effect that an order for disclosure of a source has on the exercise of that freedom, such a measure cannot be compatible with Article 10 unless it is justified by an overriding requirement in the public interest," it said.
"Interbrew's interests in eliminating, by proceedings against [the unknown leaker], the threat of damage through future dissemination of confidential information and in obtaining damages for past breaches of confidence were, even if considered cumulatively, insufficient to outweigh the public interest in the protection of journalists' sources," said the ruling. "The Court finds that there has been a violation of Article 10 of the Convention."
“This is a vindication of our longstanding belief that the original judgment in the British courts was wrong," said Martin Dickson, deputy editor of the FT, according to that paper. "It is an important day for press freedom.”