The 17-year-old is the sixth to be detained by police over the attack on Friday 15 September.
48 and 30-year-old men arrested in Newport over London Underground explosion.
Police investigating the attack, which left 30 people injured, have detained a third man.
Teenage suspect reportedly had frequent run-ins with the police and authorities.
Security is on high alert following the latest attack.
An 18-year-old man believed to be from Iraq is being questioned by police.
The downgrading means authorities no longer believe an attack is imminent.
A 21-year-old man has been arrested in Hounslow.
Ronald Jones, 88, and Penelope Jones, 77, were awarded MBE's for their work with refugees.
Phrase became the third most popular topic on Twitter across London a day after the explosion.
London's Metropolitan Police have issued a statement regarding the "significant arrest" of an 18-year-old.
Military deployed to London streets in wake of Tube bombing.
The man said he was carrying the weapon 'for his own protection' after the terror attack on Parsons Green.
Prime Minister Theresa May said another attack is imminent but urged the public to 'go about their business as usual'.
In a statement released to the Amaq news agency, Isis said 'a detachment' from their group had carried out the attack.
Trump claimed the "loser terrorist" suspect was being monitored by police prior to the attack.
Police confirm 18 people have been injured.
Several people injured after suspected explosive device detonated in West London.
Victims describe a fireball that engulfed the carriage and left people with facial burns.