The threat from terrorism to the UK remains real and serious. An attack could take place at any time and any organisation could be directly or indirectly affected. Acts of terrorism vary in scale and purpose. Some aim merely to inflict superficial damage or cause public distress to draw attention to a particular cause. But others can be more violent and indiscriminate with far-reaching consequences.
The most significant threat comes from international terrorism with its ambitions to mount high impact attacks combining mass casualties with substantial disruption to vital services such as energy, transport and communications. This is a threat that is different in scale and intent to any that the UK has faced before.
Northern Ireland-related terrorism also continues to pose a serious threat. Despite a peace process that has been active for several years, dissident Irish republican terrorists have continued to attack economic and political targets.
Whilst co-ordinated anti-terrorism operations have achieved considerable success, the intelligence accumulated during recent police investigations and subsequent trials reveals that terrorist groups, both at home and abroad, continue to target UK citizens, businesses and interests.
Al Qaida (AQ)
While the UK has faced a variety of terrorist threats in the past, a unique combination of factors - namely the global reach, capability, resilience, sophistication, ambition and lack of restraint of Al Qaida (AQ) and associated groups means the UK currently faces a threat on a scale not previously encountered.
Al Qaida and networks associated with it are intent on attacking UK, US and other Western interests, as well as replacing Islamic regimes which are not deemed pious enough.
Many of these networks are loose-knit, operating without a conventional structure and with connections across the world, bound by shared extremist views or experiences. Some of these networks are centrally guided by Al Qaida, others are autonomous, but both work to carry out terrorist attacks.
Al Qaida and its associated networks remain capable of carrying out major terrorist attacks, such as those on London in July 2005. A number of its senior leaders and many trained terrorists remain at large. The threat from Al Qaida and associated networks is therefore likely to persist for some time.
Northern Ireland-related terrorism
Republican Terrorist Groups present a serious threat to UK interests.
These groups reject the Belfast Agreement (the Good Friday Agreement), continue to engage in acts of terrorism in Northern Ireland and aspire to attack targets in Great Britain.
The Provisional Irish Republican Army (PIRA) has not mounted a terrorist campaign since 1997. Its associated political party, Sinn Féin, is engaged in the Northern Ireland political process.
Other extremist groups, unrelated to the Northern Ireland situation, may aspire to campaigns of violence but lack developed terrorist capability.
Example methods of attack
In terms of physical attacks, most terrorist bombs are known as improvised explosive devices (IEDs) and can be categorised either by their means of delivery (vehicle-, person-borne etc.) or by their content (chemical, biological, radiological etc.).