Prime Minister Theresa May has condemned the "cowardly" Parsons Green terror attack, saying it involved a device "clearly intended to cause significant harm".
She said the terror threat level would remain at severe - meaning an attack is highly likely - but would be kept under review as the investigation progresses.
Speaking in Downing Street after chairing a meeting of the Cobra emergency committee she said there would be an increased armed police presence on the transport network in London.
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"The police and the security services are obviously doing all the work to discover the full circumstances of this cowardly attack," she said.
All you need to know about what is known - and what is not - about the bomb attack on a London Tube train.
The installation of CCTV cameras on the the Tube could play a key role in finding the perpetrator of the Parsons Green explosion.
Theresa May rebuked the US president after he implied the London tube blast was carried out by suspects "in the sights" of Scotland Yard.