The Joint Terrorism Analysis Centre raised the threat level last night – meaning an attack not only remains highly likely but a further attack may be imminent.
Members of the public may see an increased police presence, including armed units, at key locations and events across the country.
Already this has been seen in Brighton and Hove, with armed officers at Brighton Railway Station this morning.
National Policing Lead for Counter-Terrorism, Assistant Commissioner Mark Rowley said: “The investigation into the terrorist attack in Manchester is large scale, fast moving and making good progress. There has been an arrest and there are currently multiple searches and other activity taking place as I speak.
“However at this stage it is still not possible to be certain if there was a wider group involved in the attack; 24 hours in we have a number of investigative leads that we are pursuing to manage the ongoing threat.
“This concern has led to the independent Joint Terrorism Analysis Centre to raise the threat level to “critical” this evening. It means that their assessment is not only that an attack remains highly likely but a further attack may be imminent.
“The public would expect the police to do everything possible to prevent further attacks and keep them safe. We are flexing our resources to increase police presence at key sites, such as transport and other crowded places and we are reviewing key events over the coming weeks.
“As the Prime Minister has already said I have asked for support from the military to be deployed alongside the police. This will free up armed officers from certain guarding duties to release our officers to support the wider response.
“This is part of an agreed and well -rehearsed plan and military personnel will remain under the command and control of the police service.
“At this time we are asking the public to remain calm but alert. If you see anything that causes you concern, then let us know immediately, if you have suspicions about someone’s behaviour call us.
“Look out for anything that seems out of place, unusual or doesn’t seem to fit in with day-to-day life. It may be nothing but if you see or hear anything that could be terrorist related trust your instinct no matter how small then ACT and call 999.”
Latest guidance on recognising the terrorist threat and remaining vigilant can be accessed via www.nactso.gov.uk.