US President Donald Trump has been slammed for his response to Friday’s London tube attack, after suggesting UK police may have missed an opportunity to prevent ‘a loser terrorist’ from striking.

The London Ambulance Service says 18 people have been taken to hospitals with injuries, but none were thought to be in a life-threatening or serious condition. A further four people arrived at hospital themselves, reports www.stuff.co.nz.

Trump tweeted: ‘Another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!’

He later added: ‘Loser terrorists must be dealt with in a much tougher manner. The internet is their main recruitment tool which we must cut off & use better!’

The suggestion drew swift rebuke from UK Prime Minister Theresa May.

‘I never think it’s helpful for anybody to speculate on what is an ongoing investigation,’ she said, when asked about Trump’s tweets.

Nick Timothy, a former chief of staff to May, blasted Trump.

‘True or not - and I’m sure he doesn’t know - this is so unhelpful from leader of our ally and intelligence partner,’ Timothy tweeted.

Trump told reporters on Friday that he had been briefed on the explosion, but did not provide further details. He said he planned to call May and described the bombing as a ‘terrible thing.’ He said ‘we have to be very smart. We have to be very, very tough’.

Trump has lobbed trans-Atlantic criticism after previous attacks.

In June, after attacks in the London Bridge area that claimed seven lives, London Mayor Sadiq Khan warned locals not to be alarmed by the large presence of armed officers on the capital’s streets. Trump accused the mayor - whom he has long sparred with - on Twitter of suggesting there was ‘no reason to be alarmed’ by the attack itself.

Trump’s rapid reaction on Friday contrasts with a statement he made defending his muted response to the violence in Charlottesville ,Virginia last month, when he said he wanted to ensure he had the facts and not make a ‘quick statement’.

Trump also used the incident to defend his travel ban targeting six mostly Muslim nations.

He tweeted: ‘The travel ban into the United States should be far larger, tougher and more specific-but stupidly, that would not be politically correct!’