Reasonable worst case scenario

The National Risk Register 2025 says that risks in it and in the National Security Risk Assessment "are assessed as 'reasonable worst-case scenarios'". It defines these as "the worst plausible manifestation of that particular risk (once highly unlikely variations have been discounted)", produced "to enable relevant bodies to undertake proportionate planning". The register is explicit that the scenarios "are not a prediction of what is most likely to happen". Relevant bodies consider many variations of scenarios at the planning stage, and additional scenarios are given for a risk where they would produce substantially different impacts or need significantly different planning.

Each scenario is the unit that gets scored. Likelihood is presented as the percentage chance of the reasonable worst case scenario occurring at least once in the assessment period, five years for non-malicious risks and two years for malicious risks. Impact is scored on the scope, scale and duration of the harm the scenario could foreseeably cause.

The register gives examples of the approach changing. Before 2023 the pandemic scenario was based on an influenza-like illness; lessons from COVID-19 led to reshaping scenarios "to include multiple and varied scenarios when assessing the impacts of risks". The national power failure scenario is a total failure of the National Electricity Transmission System, causing a nationwide loss of power without warning.