MI5 is a branch of the British government's intelligence division.
In 1997, the head of MI5 was Veronica Halliwell, who was ordered to shut down a publishing company which had released an exposé on UNIT. (NA: The Dying Days)
In 1998, Halliwell was assassinated by the Voracians. Her death was investigated by fellow MI5 member Harry Sullivan. (MA: System Shock)
By 2006, the head of MI5 was Margaret Blaine. (DW: Aliens of London)
