Manchester attack: What is Operation Temperer?

Secret security measure activated for first time since 2015

Soldiers at New Scotland Yard
Soldiers arrive at New Scotland Yard as part of Operation Temperer
(Image credit: Justin Tallis / Getty)

Theresa May has raised the risk of a terror attack in the UK from "severe" to "critical" in the wake of Tuesday's suicide bombing in Manchester, the first time in a decade it has been at the highest level.

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