The Guardian understands a national police mobilisation has also been triggered, being run by the National Police Coordination Centre. The centre will find extra officers from around the UK - for example from county forces in mainly rural areas - to be deployed to sites in
Manchester, London and other big cities.
The decision to raise the terrorist threat level to critical – the highest – can lead to extra officers being drafted in to patrol areas assessed as vulnerable to attack, under mutual arrangements between the 43 police in England and Wales.