Up to 23,000 people have been considered "subjects of interest" in the past by the British counter-terrorism agencies, according to the new figures announced after the deadly Manchester terror attack.

Security sources, however, did not clarify which period these figures of potential threats covered.

According to the Telegraph, the MI5 domestic security service has been dealing with around 500 active investigations, including some 3,000 "subjects of interest" at the present.

Britain's anti-terror efforts have become a much-debated issue after the Manchester suicide bomber Salman Abedi was identified to be a "former subject of interest" to the MI5.

Sources claimed that the Daesh terrorist was well known to the counter-terrorism agencies, as they were informed about Abedi posing a threat in the past five years for at least five times, the Telegraph reported.

A senior government official previously admitted that a total of 18 plots had been hindered in Britain since 2013, including five since Westminster attack in March, which led to the death of 5 people and injured 49 others.

Last Monday, suicide bomber Salman Abedi targeted concert goers in Manchester, killing 22 people and wounding 120 others.