LONDON (BLOOMBERG/AFP) - Mr Donald Trump is raising hackles in London again.

Tweets from the United States President following a terrorist attack on Friday (Sept 15) prompted an immediate backlash. Among his critics was Mr Nick Timothy, until recently one of British Prime Minister Theresa May's most senior aides.

Mr Trump was accused of betraying intelligence details by saying those responsible for an explosion on an underground train "are sick and demented people who were in the sights of Scotland Yard. Must be proactive!"

It was not immediately clear if the people behind the bomb attack were indeed previously known to British law enforcement. But if it were the case, then Mr Trump has apparently revealed the detail before authorities in Britain made the information public.

"True or not - and I'm sure he doesn't know - this is so unhelpful from leader of our ally and intelligence partner," tweeted Mr Timothy, who was Mrs May's chief of staff until the June general election and worked for her as a senior adviser when she was home secretary.

Mrs May, when asked directly about Mr Trump's tweets, responded in a broadcast interview: "I never think it's helpful for anybody to speculate about what is an ongoing investigation."

Former Conservative lawmaker Ben Howlett echoed the comments.

"It is highly unhelpful/dangerous and inappropriate for an ally to make announcements that share intelligence and undermine investigations," he said on Twitter.

Others pointed out that any other members of a potential terror cell may have been alerted that they had been under surveillance if the president was basing his comments on a confidential security briefing.

There was talk early this year of suspending transatlantic security briefings after details of the investigation into the bombing of an Ariana Grande concert in Manchester, northern England, which killed 22 people, were leaked to the media by US authorities.

Mr Trump also sparked anger after van and knife attacks on London Bridge when he used Twitter to criticise Mayor Sadiq Khan's response just days before the June general election.

Mrs May's reluctance to stand up to him over those tweets led to criticism of her during the campaign.