The high court judgment allowed the Mail & Guardian and its former reporter, Adriaan Basson, to protect the identity of sources who provided information that formed part of the basis for a series of articles alleging graft in massive prisons tenders.
Bosasa is suing the newspaper for defamation, and had demanded that the M&G and Basson provide unredacted versions of documentary evidence that featured in the investigation of its relationship with senior department of correctional services officials as part of the legal discovery process ahead of trial.
Judge Tsoka ruled, however, that protecting the anonymity of sources was crucial to to the ability of the press to carry out its democratic function, which includes revealing corruption: "It is essential that in carrying out this public duty for the public good, the identity of their sources should not be revealed", he said.
"This essential and critical role of the media, which is more pronounced in our nascent democracy, founded on openness, where corruption has become cancerous, needs to be fostered rather than denuded," he added.
Handing down his ruling denying leave to appeal, Judge Tsoka said on Thursday that in his view no court would come to a different conclusion.
Bosasa still has the option to petition the Supreme Court of Appeal directly for leave to appeal.
M&G editor-in-chief Nic Dawes said he was pleased by the decision: "Judge Tsoka's original finding was an important precedent on source protection, bringing our common law in line with the constitutional recognition of press freedom. There can be no investigative journalism without the brave people who blow the whistle, and we must be able to stand by our commitment to keep confidential their identities.
"If Bosasa wishes to continue with their defamation suit, which we believe is without merit, we hope that they will now do so on the basis of the facts, rather than using it as an opportunity to hunt for those who were prepared to assist in revealing those facts".