Iraqi teenager charged over London Underground attack: police

Ahmed Hassan, who was arrested the morning after the attack on September 15, was also charged with use of the explosive, triacetone triperoxide (TATP), a statement said.

The teenager, who is believed to be an Iraqi orphan, lived with British foster parents in the London suburb of Sunbury. The house was one of several addresses searched by police after the attack.

Earlier on Friday, London police chief Cressida Dick said the bomb set off on a packed rush-hour tube train was "very, very dangerous" and had been packed with shrapnel – 30 people were injured in the attack.

"Thank goodness nobody was killed at Parsons Green. It must have been absolutely horrific for the people in the train. As you know, many people suffered injuries," Dick told LBC radio.

"That was a very, very dangerous bomb. It partially detonated. It had a large quantity of explosive and it was packed with shrapnel."

The explosion at Parsons Green station in south-west London, was Britain's fifth terror attack in six months, and was claimed by the Islamic State group, although both British and US officials have cast doubt on the statement, saying there was no evidence any recognised militant group had ordered or organised the bombing.

The bomb had been hidden in a large white bucket and it apparently failed to detonate fully during the morning rush hour but resulted in what witnesses described as a "fireball".

Some commuters suffered burns, while others were injured in the stampede to the exit that ensued. 

Hassan was arrested in the southern English port of Dover shortly before 8am the following day.

Three other men, aged 17, 25, 30, remain in custody in connection with the attack. Two men aged 21 and 48 were released with no further action on Thursday.

Agencies contributed to this report.