- Video report by ITV News Washington Correspondent Robert Moore
Theresa May has rebuked US President Donald Trump after he implied the Parsons Green blast was carried out by people "in the sights" of Scotland Yard.
Following a meeting of the Government's Cobra emergency committee, the prime minister said: "I never think it's helpful for anybody to speculate on what is an ongoing investigation."
She also added that police and security services were still working to identify those responsible for the attack on the packed commuter train.
The explosion at the tube station in south-west London involved an improvised explosive device and has been declared a "terrorist incident" by police.
Writing on Twitter, Mr Trump described suspects as "sick and demented" and claimed they were "in the sights of Scotland Yard".
The Metropolitan Police said the US president's comments were "unhelpful" and "pure speculation".
When asked about the suspected terror attack outside the White House, Mr Trump said: "It's a terrible thing. It just keeps going and going, and we have to be very smart, we have to be very, very tough.
"Perhaps we are not nearly tough enough."
"It's just an absolutely terrible thing. In fact, I'm going to call the Prime Minister [Mrs May] right now," he added.
Twenty-two people are in hospital after suffering burns in the incident which took place at around 08.20am on Friday morning.
Children are reportedly among the victims, but none of their injuries are thought to be life-threatening.
The blast comes with the UK on high alert following four terrorist attacks so far this year.
The incident also comes a day after figures revealed terror-related arrests in Britain have hit a new record high, with suspects held at a rate of more than one every day.