The UK is on a critical terror alert with military troops set to bolster police forces after the Prime Minster raised the threat level to its highest possible rating.
Theresa May made the sombre announcement in a live television statement from Downing Street on Tuesday evening.
The move comes amid fears the Manchester attacker, Salman Abedi, did not act alone.
It means a terrorist attack is considered imminent.
"It is a possibility that we cannot ignore that there is a wider group of individuals linked to this attack," Mrs May said.
Monday night's attack at a concert by US pop star Ariana Grande left 22 people dead, including an eight-year-old girl, and dozens injured.
Raising the level to critical means that military personnel could be deployed to support armed police officers - part of a plan known as Operation Temperer.
"The change in the threat level means there will be additional support available to the police as they work to keep us safe," she said.
"Operation Temperer is now in force."
The UK's international terrorism threat level is set by the Joint Terrorism Analysis Centre (JTAC), based in MI5 headquarters in London.
This is the first time in a decade that the assessment has been placed at the highest level.
It has only been at critical twice in nearly 11 years - once in August 2006 and again in June 2007.
On both occasions the assessment was lowered after a few days.