Official Secrets Act
The Official Secrets Act was legislation which protected official state secrets in the United Kingdom on Earth. Signing the Official Secrets Act constituted an agreement not to reveal certain secret state matters. Breach of the Act was a jailable offence.
References
The Brigadier Alistair Gordon Lethbridge-Stewart appealed to the Act to count on the discretion of Dr. Humphrey Cook about the mysterious events at the Newton Institute. (TV: The Time Monster)
An oblivious Brigadier asked the Fifth Doctor if he had signed the Act, since he was speaking freely about a secret organisation as the UNIT. (TV: Mawdryn Undead)
Liz Shaw was reluctant to answer James Stevens' questions in part because she had signed the Official Secrets Act. (PROSE: Who Killed Kennedy)
Mike Smith, sergeant in the RAF, was placed under close arrest on suspicion of offences contrary to the Official Secrets Act. In fact, he was working as an informant for the fascist group the Association, allied to renegade Daleks. (TV: Remembrance of the Daleks)
The relationship between Alistair Gordon Lethbridge-Stewart and his daughter Kate suffered due to his inability to explain his work because of the Official Secrets Act. (HOMEVID: Downtime, PROSE: Downtime)
In the aftermath of a soldier's death during an incident with the Master at Devil's End, Brigadier Lethbridge-Stewart carefully told the soldier's father all he could about his son's death without breaching the Act. (AUDIO: Degrees of Truth)
Details of the petrol-bombing of a McDonald's restaurant in Kent were suppressed under the Official Secrets Act. (PROSE: Cat's Cradle: Warhead)
Many employees of Scala, a scientific company Sarah Jane Smith investigated, were past members of an organisation which dissolved when many of its members were jailed for breaches of the Official Secrets Act. (AUDIO: Mirror, Signal, Manoeuvre)
Lauren Hudson signed the Act after UNIT removed a Viyran virus from her body, saving her life. (AUDIO: Urgent Calls)
The Sixth Doctor claimed to have signed the Official Secrets Act. (AUDIO: Criss-Cross)