Isis have claimed responsibility for the explosion on the London Underground at Parsons Green.
Twenty-nine people were hurt after an Improvised Explosive Device (IED) caused a “fireball” on a Tube carriage during rush hour.
A manhunt for the perpetrator is ongoing and police have reportedly identified a suspect.
According to a translation by Middle East observer Hassan Hassan, a message on the Isis’s propaganda page, Amaq, said: “A security source to Amaq: The bombing of the IED in the London tube was carried out by a unit affiliated to the Islamic State.”
Experts have urged caution over the veracity of the claim.
Shiraz Maher, an expert on radicalisation at Kings College London, said it was unusual for such announcements to be made while the suspect was still at large.
Parsons Green explosion – in pictures
He said the declaration was a “break from the norm” and was “significant”.
Others said it was a sign of desperation.
Separate unconfirmed reports said the group claimed there were other devices that had not detonated.
The claim of responsibility came as the UK’s terror alert level was raised to critical, with military troops set to bolster police forces as they hunt the bomber.
Theresa May raised the threat level to the highest possible rating, meaning another attack is expected imminently.
The Prime Minister said military personnel would replace police officers “on guard duties at certain protected sites which are not accessible to the public”.
She said: “The public will see more armed police on the transport network and on our streets, providing extra protection.
“This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses.”