D-notice

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

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)