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.
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)
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)