Three more men have been arrested as police continue to investigate whether Manchester Arena bomber Salman Abedi acted alone.
Abedi's 23-year-old brother has also been arrested.
Abedi killed 22 and injured 64 when he blew himself up outside an Ariana Grande concert on Monday night. An off-duty policewoman is among the dead, the BBC understands.
The UK terror threat level is now up to its highest level of "critical".
This means more attacks may be imminent.
Military personnel are being deployed to protect key sites.
The Palace of Westminster has been closed to the public following police advice, and will not re-open until further notice, its website said.
Home Secretary Amber Rudd said: "[Monday's attack] was more sophisticated than some of the attacks we've seen before, and it seems likely - possible - that he wasn't doing this on his own."
BBC security correspondent Frank Gardner said that the bomber is thought to have been a "mule", using a device built by someone else.
The victims include Nell Jones, 14, eight-year-old Saffie Roussos, Alison Howe, Lisa Lees, Jane Tweddle-Taylor, 50, Martyn Hett, 29, and Olivia Campbell, 15, .
Kelly Brewster, 32, John Atkinson, 28, Georgina Callander - thought to be 18 - and Marcin and Angelika Klis, a Polish couple from York, have also been named.
An off-duty police officer - who has not yet been named - was also among those killed, the BBC understands. Her husband is believed to be critically ill and their two children were also injured.
Of the 64 injured, 20 are in a critical condition, and some have lost limbs. Twelve of them are known to be children.
Several people are still missing, including Eilidh MacLeod, 14, from Barra in the Outer Hebrides, Chloe Rutherford, 17, and Liam Curry, 19.
Eilidh's friend, Laura MacIntyre, 15, who was also reported as missing, was later identified as one of the seriously injured in a Manchester hospital.
The police believe they have now identified all those killed and informed their families.
They will formally name the victims after post-mortem examinations, a process likely to take four or five days.
A hotline has been set up for people concerned about loved ones - 0800 096 0095.(Source: BBC)