Britain's terror threat has been raised to critical by Prime Minister Theresa May after fears that a "wider group of individuals" could have been involved in Monday night's (May 22) attack in Manchester.

The Prime Minister raised the threat to the highest possible rating amid fears that Manchester Arena attacker Salman Abedi did not act alone.

Mrs May announced another attack is expected imminently in a televised statement from Downing Street on Tuesday night (May 24).

The raised level follows the suicide bombing at the end of an Ariana Grande concert which left 22 people dead including an eight-year-old girl.

Metropolitan Police confirmed ncreased police numbers and operations would take immediate affect in London as a response to the attack on Monday (May 22).

Commander Jane Connors, who is leading the police operation, said: "The reality is that we must be prepared to be able to respond to and deal with armed and deadly attackers, so we must be in a position to respond with firearms officers who will use force to stop those attackers in their aim.

"That is why is we have increased the number of firearms who are on duty, both out walking and in roaming patrols, at fixed points and carrying out a range of operations.

Police at Manchester Arena after reports of an explosion at the venue during an Ariana Grande gig.

"We are using every tactical option - not just through the use of armed officers but ongoing work by the Counter Terrorism Command; working with partner agencies and gathering community intelligence."

The Islamic State terror group claimed responsibility for the attack, which involved a home-made device detonated by suicide bomber Abedi in the Manchester Arena foyer.

Home Secretary Amber Rudd disclosed the intelligence services had been aware of British-born Abedi, who is reported to have recently returned to the UK from a visit to Libya.

She told Sky News: "We do know that he was known up to a point to the intelligence services."

The Prime Minister said Operation Temperer, which allows military personnel to take to the streets, is now in force.

This was backed by Assistant Commissioner and Head of National Counter Terrorism Policing Mark Rowley who explained military troops will also be deployed to support armed police officers.

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He said: "As the Prime Minister has already said this evening 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."

Counter-terrorism agencies mounted a massive inquiry into the atrocity, the worst terrorist attack since 52 innocent people were killed in the July 7 bombings in London in 2005.

Many of the 59 people hurt in the attack were treated for life-threatening injuries and 12 of those rushed to hospital were children.

Among the first victims to be named were eight-year-old Saffie Roussos from Leyland and teenagers Olivia Campbell, 15, from Bury and Georgina Callander from Chorley.

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