D-notice

A  was an act that could be issued by the government or military. It was issued in cases of national security to keep an investigation from the press or to shut down an investigation.

History
Greg Sutton mentioned to Petra Williams before he went to be interviewed by James Stevens about the Inferno Project: "We don't pay much attention to pommie D-notices where I come from." He also figured Stevens wouldn't believe a word of the story anyway — which was indeed the case. (PROSE: Who Killed Kennedy)

Brigadier Lethbridge-Stewart issued a D-notice regarding the murder of Cheng Teik, the Chinese delegate to the First World Peace Conference. (TV: The Mind of Evil)

Sarah Jane Smith and the Third Doctor were investigating the strange events behind Isaac's death when Sarah's editor received a D-notice. The Brigadier and the Doctor tried to convince the higher powers to rescind the D-notice, but gave in when they learned it had been issued by a future version of the Doctor. (PROSE: An Overture Too Early)

In 2016, UNIT issued a D-notice to keep the media away from San Francisco when the gargoyles first began appearing in the city to the disappointment, but utter lack of surprise, of the Ninth Doctor and Mickey Smith. (COMIC: The Transformed)

After Jenny's bowship had crashed into, and demolished, the Terrance Dicks Library, Kate Stewart issued a D-notice to keep the BBC away from the crash site. (COMIC: The Lost Dimension)