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MI5 protects the uk against threats to national security

The role of 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:

The national security strategy – a strong Britain in an age of uncertainty

The strategic defence and security review: securing Britain in an age of uncertainty

Threats to national security

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


Terrorist groups use violence and threats of violence to publicise their causes and as a means to achieve their goals.

More on Counter-Terrorism


The UK is a high priority espionage target. Many countries actively seek UK information and material to advance their own agendas.

More on Counter-Espionage



A wide range of hostile actors target the UK in cyberspace, including foreign states, criminals, “hacktivist” groups and terrorists.

More on Cyber


A number of countries continue to develop weapons of mass destruction programmes, posing a potential threat to UK security.

More on Counter-Proliferation

Protective security

The provision of protective security is one of MI5’s main activities – we work to ensure there is enough protection for the critical parts of the UK’s national infrastructure. The Centre for the Protection of National Infrastructure (CPNI) carries out much of this work, and is accountable to the Director General of MI5.
CPNI aims to reduce the vulnerability of the national infrastructure to terrorism and other threats. The national infrastructure consists of key assets – physical and electronic – that are vital to the continued delivery and integrity of essential services such as energy, communications, transport and water. The national infrastructure faces threats from international and domestic terrorism, espionage and other hostile foreign activity. To counter these, CPNI provides authoritative expert advice to organisations across the national infrastructure, covering physical and personnel/people protective security. All the advice is informed by access to intelligence and information about the threats. CPNI works closely with the National Cyber Security Centre which provides advice on cyber security.

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)

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