MI5 - Security Service

What we do

MI5 protects the UK against threats to national security

The role of the MI5, as defined in the Security Service Act 1989, is "the protection of national security and in particular its protection against threats such as terrorism, espionage and sabotage, the activities of agents of foreign powers, and from actions intended to overthrow or undermine parliamentary democracy by political, industrial or violent means".

Our work is guided by the Government's overall strategy to counter threats to the UK's national security. For more information on this strategy, see:

Threats to national security

Regnum defende - the MI5 crest

The main threats to national security that MI5 counters are terrorism, espionage, cyber threats and the proliferation of weapons of mass destruction.

Counter-terrorism and protective security

MI5's principal activity is the fight against terrorism. This includes both international and Northern Ireland-related terrorism, and the provision of protective security advice in support of that task.

Resources dedicated to this area of work has grown in the last few years owing to the increased threat from international terrorism.

The fight against international terrorism accounts for 65% of our overall expenditure. We continue to do a substantial amount of work to counter Northern Ireland-related terrorism, and this accounts for around 15% of our resources (see Funding and resource allocation).


Espionage is still a concern. Although the nature of the threat has changed since the end of the Cold War, the intelligence services of Russia, China and other countries continue to work against UK interests at home and abroad.


The proliferation of weapons of mass destruction poses a serious potential threat to the UK's national security. MI5 has played a part in combating this threat since 1992, supporting the work of other Government organisations.

Other work

During much of the 20th century, subversion was a major concern for MI5. This threat diminished sharply following the end of the Cold War. We no longer undertake counter-subversion work, and would only resume doing so if our monitoring of emerging threats suggested an increase in the subversive threat.

We became involved in supporting police and law enforcement investigations of serious crime in 1996. This activity was suspended in 2006 so that we could concentrate on counter-terrorism. Work on serious crime is now carried out by the National Crime Agency (NCA) Link to external website..

Key points

Main responsibilities:

  • Counter terrorism and protective security
  • Counter espionage
  • Counter proliferation
  • Role is defined in Security Service Act 1989.

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