In spelling out the differences between MI5 and MI6 one senior official once said "We are the spycatchers, they are the spies." According to the official MI5 internet site the Security Service – the official name for MI5 (originally, Military Intelligence, Desk Five) – is the security intelligence agency of the UK. Its functions are to protect security, to safeguard the economic well-being from threats from overseas and to support the police in the prevention and detection of serious crimes. MI5’s key responsibility is to investigate and counter covertly organized threats. These include: terrorism, espionage and the proliferation of weapons of mass destruction. Put another way: "We cover everything from anarchists in the attic, reds under the beds and subversives down in the cellar."

Things haven’t always been so transparent. MI5 has only been officially recognized since the Security Service Act of 1989, although it is fair to say that before this it was probably the worst-kept secret in 20th century British history.

In 1952 Prime Minister Winston Churchill, gave responsibility to the home Secretary, Sir Maxwell Fyfe, who issued a directive setting out MI5’s and its Director General’s roles until they were placed on a statutory footing in 1989.

MI5 is more commonly known by their shortform official mailing address, "Box".

On the rivalry between MI5 and MI6

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