The UK threat level is an official assessment of the likelihood of a terrorist attack in the UK. It helps government agencies, businesses, and the public understand the current risk and take appropriate security measures. The system is maintained by the Joint Terrorism Analysis Centre (JTAC) and MI5 (the Security Service).
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There are five levels of threat:
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Low: An attack is highly unlikely. No significant threat seen.
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Moderate: An attack is possible, but not likely. Some concern, but no intelligence of an imminent threat.
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Substantial: An attack is likely. There is a credible risk.
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Severe: An attack is highly likely. Strong intelligence of a serious threat.
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Critical: An attack is highly likely in the near future. Specific intelligence suggests an imminent attack.