WASHINGTON (AFP, BLOOMBERG) - US President Donald Trump on Monday (Feb 5) attacked Britain's public healthcare system in comments that are likely to call his much-delayed visit to the country further into doubt.

"The Democrats are pushing for Universal HealthCare while thousands of people are marching in the UK because their U system is going broke and not working," he wrote in an early morning tweet.

"Dems want to greatly raise taxes for really bad and non-personal medical care. No thanks!"

The tweet came after thousands of people marched through central London on Saturday in support of the National Health Service, which is straining under the weight of winter demand.

NHS staffing levels have been in crisis for months, an issue made worse by a winter flu outbreak.

The Democrats are pushing for Universal HealthCare while thousands of people are marching in the UK because their U system is going broke and not working. Dems want to greatly raise taxes for really bad and non-personal medical care. No thanks! — Donald J. Trump (@realDonaldTrump) February 5, 2018

Despite its current woes, the NHS, which was created after World War II, is a revered institution and Mr Trump's comments are likely to stoke resentment.

“NHS may have challenges but I’m proud to be from the country that invented universal coverage – where all get care no matter the size of their bank balance,” said UK Health Minister Jeremy Hunt in a tweet Monday.

The "special relationship" between Britain and the United States has shown some signs of strain since Mr Trump came to office a year ago.

British Prime Minister Theresa May was the first foreign leader to visit Mr Trump following his inauguration in January last year, when she invited him to make a state visit to Britain, hosted by Queen Elizabeth II.

The trip has been delayed, however, and Mr Trump recently pulled out of a plan to open the new US embassy in London, a move British officials blamed on threats of mass protests.

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Mr Trump has also angered the British with previous controversial tweets, including retweeting an extremist group's anti-Muslim propaganda and sparring with London mayor Sadiq Khan following a terror attack.

It is not certain why Mr Trump chose to attack the NHS two days after the London protests.

Mr Trump's own attempts to reverse his predecessor's healthcare reform, known as Obamacare, twice ended in failure, before his party succeeded in eliminating a key element - the so-called "individual mandate," as part of tax reform.

The measure required individuals to buy coverage as a way to lower costs by ensuring that healthy people were part of insurance pools.

One possible explanation for Mr Trump's criticism was an appearance by Brexit champion Nigel Farage, a personal friend of the president, on Fox News earlier Monday in which he questioned the feasibility of universal healthcare and blamed the NHS's predicament on immigrants.