ONLY 155 characters long, Donald Trump's Twitter barb aimed directly at Prime Minister Theresa May will likely make many in Westminster shudder when they check their phones on Thursday morning - and probably not for the first time.

The president has caused diplomatic headaches for UK officials on several previous occasions, from backing Nigel Farage as ambassador to Washington to attacking London Mayor Sadiq Khan, all via brief messages on his favourite social media platform.

In his latest missive, fired off at around 1am GMT, Mr Trump told Mrs May "don't focus on me" after Downing Street said the president had been "wrong" in his use of Twitter to share anti-Muslim videos by the far-right group Britain First.

He told the PM: "Focus on the destructive Radical Islamic Terrorism that is taking place within the United Kingdom. We are doing just fine!"

The White House's current resident is famed for not mincing his words, although public exchanges between US and UK leaders in recent decades have been closely watched to determine the health of the so-called "special relationship".

Often swift to tweet his condemnation of terrorist attacks, Mr Trump recently drew criticism from Mrs May following the bombing of a tube train at Parsons Green in London.

He 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!"

Mrs May said it was "unhelpful" to speculate on what is going on in an investigation.

In the wake of deadly attacks in London in June, Mr Trump's reaction drew the ire of the mayor of London, who he singled out on Twitter.

"At least 7 dead and 48 wounded in terror attack and Mayor of London says there is 'no reason to be alarmed!," the president wrote.

Mr Khan told The Guardian: "We're not schoolchildren. He's the president of the United States, so I'm unclear what his beef is with me."

The potential for Twitter-enabled diplomatic trouble became apparent soon after Mr Trump was elected in November 2016.

Before he had even stepped in the White House, Downing Street had to publicly contradict the president-elect's suggestion that Mr Farage should be the UK's representative in Washington.

The president tweeted: "Many people would like to see @Nigel_Farage represent Great Britain as their Ambassador to the United States. He would do a great job!"

Number 10 was forced to say there was "no vacancy" and praised Britain's incumbent ambassador, Sir Kim Darroch.

"We already have an excellent ambassador to the US," a spokesman said.