The UK Threat Levels are the only one alert states that have been in use since 1 August 2006 by the British government to warn of forms of terrorist activity. Before then a colour-based alert scheme known as BIKINI state was used. The response indicates how government departments and agencies and their staffs should react to each threat level.
Since 24 September 2010, the Home Office has reported three different categories of terrorist threat:
Threat from international terrorism.
Terrorism threat related to Northern Ireland in Northern Ireland itself.
Terrorism threat related to Northern Ireland in Great Britain.
The Joint Terrorism Analysis Centre (JTAC) is responsible for setting the threat level from international terrorism and the Security Service (MI5) is responsible for setting both threat levels related to Northern Ireland.