Theresa May has condemned the 'sickening cowardice' of the terrorist attack on a teen pop concert in Manchester on Monday night, adding her 'thoughts and prayers' were with the victims.

The Prime Minister confirmed the police knew the identity of the killer as she gave a statement on the steps of Downing Street on Tuesday morning.

May, who had just chaired a two-hour meeting of the security panel Cobra, will travel to Manchester this afternoon to meet with leaders and express her sympathy.

At least 22 people were killed, including children, and another 59 have injuries, some of which are life threatening, May said. She will chair another meeting of Cobra later on Tuesday.

'All acts of terrorism are cowardly attacks on innocent people, but this attack stands out for its appalling, sickening cowardice, deliberately targeting innocent, defenceless children and young people who should have been enjoying one of the most memorable nights of their lives,' she said.

'We struggle to comprehend the warped and twisted mind that sees a room packed with young children not as a scene to cherish but an opportunity for carnage,' she added.

Warning of 'difficult days' ahead, the Prime Minister vowed the 'terrorists will never win'.

She said: 'At terrible moments like these it is customary for leaders,politicians and others to condemn the perpetrators and declare that the terrorists will not win.

'The fact that we have been here before and the fact that we need to say it again does not make it any less true.

'As so often while as we experienced the worst of humanity last night in Manchester we also saw the best.

'The cowardice of the attacker met the bravery of the emergency services and the people of Manchester.

'The attempt to divide us met countless acts of kindness that bought people closer together.

'In the days ahead those must be the things we remember.'