As many as 23,000 jihadist extremists currently living in the U.K. have been identified by intelligence agencies as potential terrorists.
The figures by the counter-terror agencies laid bare the scale of the potential threat in Britain.
In the wake of the Manchester suicide bombing it emerged that British authorities were grappling with 500 investigations into 3,000 individuals considered as possible threats, The Independent reported.
Security sources has confirmed a further 20,000 individuals were said to have been considered "subjects of interest" in the past.
Anti-terror efforts came under fresh scrutiny following revelations that attacker Salman Abedi, who killed 22 people and injured over a hundred at a pop concert, had been a "former subject of interest" to MI5 who was "subject to review" and was not regarded as an imminent threat.
The Independent quoted a senior Whitehall source as saying, that 18 terror plots had been foiled since 2013 in Britain, including five since the Westminster atrocity in March this year.
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The terror threat level in United Kingdom has been reduced from "critical" to "severe" after temporarily raising the level in response to Manchester Arena attack in which 22 people were killed and several others injured. The change means that an attack is highly likely and not imminently expected.
Meanwhile, pictures of the Manchester bomber on his way to the arena where he carried out Britain's worst terrorist attack in 12 years, have been released by police.
The police believe he assembled his deadly bomb in a rented Airbnb flat in a mansion block on Granby Row in the city centre, near Canal Street.
The flat is one of 17 locations searched by forensic experts in the past few days, with 14 searches continuing on Saturday night.
The police said thirteen people have so far been arrested on suspicion of terrorism offences, with 14 locations still being searched on Saturday night.
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