MI5 - Security Service

Protecting National Security

The role of the Security Service, as defined in the Security Service Act 1989 opens a new window, 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.

The term "national security" is not specifically defined by UK or European law. It has been the policy of successive Governments and the practice of Parliament not to define the term, in order to retain the flexibility necessary to ensure that the use of the term can adapt to changing circumstances.

As a matter of Government policy, the term "national security" is taken to refer to the security and well-being of the United Kingdom as a whole. The "nation" in this sense is not confined to the UK as a geographical or political entity but extends to its citizens, wherever they may be, and its system of government.

Some matters are recognised as being threats to national security, which may give MI5 a role to investigate and counter them. Where we have a reasonable belief that such a threat exists, we can carry out an investigation to determine whether or not the threat is real. Ultimately, the decision on whether the threat exists is a matter for Ministers to decide, based on evidence including intelligence material that we provide. If it is agreed that a threat does exist and is serious, MI5 works with the police and other agencies to counter it.

Some threats arise from political or industrial action or from violence that falls short of terrorism. In these cases we only investigate them if those involved specifically intend to overthrow or undermine parliamentary democracy. It is not enough that this may be the effect of their actions if those responsible do not have that intent. Such threats may endanger public order but not national security, in which case we do not generally carry out investigations. (See Domestic Extremism for more details.)

A threat to national security can arise without there being a direct threat to the UK itself. For instance, a threat to an allied nation may indirectly threaten our own national security. For this reason, we work closely with allies and partners to ensure that common threats are tackled in cooperation with others (see Partnerships).

Protecting national security involves more than just investigating and countering active threats that we know about or suspect. We also help others to protect themselves against security threats by providing security advice, and we undertake "horizon scanning" work to identify possible future security threats.

Key points

  • The term "national security" is not specifically defined by UK or European law.
  • A threat to national security can arise without there being a direct threat to the UK itself.

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