A terror plot targeting the London Underground has been uncovered after police found what they believe to be a bomb on a Tube train.
A 19-year-old man was tasered and arrested by counter-terrorism officers following the discovery of the device aboard a train outside the O2 arena in Greenwich, southeast London on Thursday (local time).
It emerged that security officials had already contacted the Ministry of Defence to warn military personnel and that the threat level for transport in the capital had been raised to “severe”.
The MoD told troops before the arrest: “It remains possible that the perpetrator may attempt to place further devices.”
British Transport Police confirmed it had increased security and would be stepping up patrols.
The incident and subsequent arrest came 11 years after the 7/7 bombings when the London Underground was targeted in a terror attack that killed 52 passengers.
Details of the suspected plot emerged in a memo sent to serving military personnel that forwarded a warning from the security agency that gives terrorism alerts.
It said the Joint Terrorism Analysis Centre “has raised the threat level for transport in London to severe: an attack is highly likely”.
It said: “This increase is in response to the discovery of a suspected viable improvised explosive device (IED) on a London Underground train yesterday.
“It is unknown who placed the device and what their motivation was.Therefore it remains possible that the perpetrator may attempt to place further devices. The threat level will continue to be reviewed as further information is received.”
It is understood the threat level for transport outside London remains unchanged at “moderate”.
The Home Office declined to say why the military had been contacted and other bodies including Transport for London and Network Rail also refused to comment.
The teenage suspect was arrested outside London Metropolitan University on Holloway Road, north London, about 14 kilometres from where the device was found in North Greenwich.
Witnesses described the young man being dragged to the ground by plain- clothed masked officers carrying machineguns.
One onlooker said police appeared to be waiting for the suspect.
The man, described as Asian in appearance with black hair, was tasered in the back.
A shop worker who did not want to be named said: “He was just walking down the street. He was definitely Asian and he definitely had black hair and I think he was in his twenties.
“Officers came from behind him and tasered him. They jumped on him. He was on the floor. They checked him. They put him in a car and took him away. I saw the whole thing from where I work.”
Specialist forensic officers will examine the suspect package over the weekend.
Scotland Yard said the arrest came “following the discovery of a suspicious item on a Tube in north Greenwich”.