The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
This means that security arrangements known as Temperer come into play, whereby the army take over from the police in guarding and protecting sensitive places, freeing up the police to be more visible and active on the streets.
Supposedly we the public will not notice the change, other than that the police will be more conspicuous.
The reason for the change is that the security services cannot be certain that the terrorist who committed the Manchester atrocity, Salman Abedi, was unconnected to a wider dangerous network of ISIS terrorists.
The upgrading of the terrorist risk assessment to the highest level has been announced by the prime minister, though the decision to do so was not taken by her or the COBRA committee she has just chaired.