Stories sourced entirely from "anonymous senior government officials" are propaganda, not journalism. The identities of the sources are hidden not to protect them from speaking out against the government, since they are in fact delivering exactly the message the government wants to get out. Instead, their identities are kept secret so that their message cannot be challenged.
It's not just me claiming this. Every journalistic organization criticizes the practice. Every set of journalistic ethics guidelines calls this unethical.
Yet, somehow it keeps happening. The latest example is the The Sunday Times, Britains largest newspaper, reporting government officials critical of Snowden. We know the story is bogus, because it quotes solely government official spouting the party line. Moreover, even if that weren't the case, it's obvious propaganda, arguing one side of the story, and not even attempting to get the other point of view from Russia, China, or Snowden himself. Snowden is often quoted in newspapers, he can't be that hard to get a hold of. Not contacting Snowden for his side is also a violation of journalistic ethics.
I point this out because there are lots of good criticisms of the story, for example, pointing out that the correct term is "MI6 officers" not "agents", and no knowledgeable government expert would make that error. But a detailed analysis of that piece isn't needed. The pure fact that it tramples all over journalistic ethics is proof enough that the story is bogus.