"The DA Notice system is a voluntary code that provides guidance to the British media on the publication or broadcasting of national security information," says the official DA-N website, http://www.dnotice.org.uk/.
DA-Notices however are not binding. According to the official website, "The system is voluntary, it has no legal authority and the final responsibility for deciding whether or not to publish rests solely with the editor or publisher concerned."
Update 1: Andrew Vallance, Secretary of The Defence, Press and Broadcasting Advisory Committee has confirmed the DA-Notice issuance to NRK.
Update 2: Alan Rusbridger, Editor in Chief of The Guardian, posted his reaction on Twitter: "Puzzled by DA Notice re #wikileaks. Overwhelming majority of t stuff not covered. "Safety + security of Brits" nothing to do w DNotice"
Update 3: Index on Censorship reacts to the DA-Notice issuance: UK issues DA-notices as US briefs allies on fresh leak.
Update 4: Guido Fawkes has published the content of the DA-Notice here.
Update 5: The Guardian's political editor Patrick Wintour on the DA-Notice: Expected WikiLeaks disclosures prompt Downing Street warning for editors. The article also quotes Alan Rusbridger as saying, "I appreciate why the DA notice might make people anxious. But, from my reading of the WikiLeaks material, only a tiny part of it is covered or relevant."
Update 6: The international media is picking up on the DA-Notice story. Die Zeit titled its report British government asks media to self-censor. CBC noted that U.K. government wants WikiLeaks media briefing.