Last July, Donald Trump said Sadiq Khan (in photo) had "done a very bad job on terrorism".

Mayor of London Sadiq Khan has compared the language used by Donald Trump to rally his supporters to that of "the fascists of the 20th century" in an explosive intervention ahead of the US Presidents state visit to the UK that begins on Monday.

Writing in the Guardian's Observer supplement on Sunday, Mr Khan condemned the red-carpet treatment being afforded to Trump who, with his wife and First lady Melania, will be a guest of Queen Elizabeth during their three-day stay.

"President Donald Trump is just one of the most egregious examples of a growing global threat. The far right is on the rise around the world, threatening our hard-won rights and freedoms and the values that have defined our liberal, democratic societies for more than 70 years," Mr Khan wrote.

"Viktor Orban in Hungary, Matteo Salvini in Italy, Marine Le Pen in France and Nigel Farage here in the UK are using the same divisive tropes of the fascists of the 20th century to garner support, but with new sinister methods to deliver their message. And they are gaining ground and winning power and influence in places that would have been unthinkable just a few years ago."

Mr Khan, who has had a feud with Trump since becoming Mayor in 2016, added: "This is a man who also tried to exploit Londoners' fears following a horrific terrorist attack on our city, amplified the tweets of a British far-right racist group, denounced as fake news the robust scientific evidence warning of the dangers of climate change, and is now trying to interfere shamelessly in the Conservative party leadership race by backing Boris Johnson because he believes it would enable him to gain an ally in Number 10 for his divisive agenda."

In May 2016, Trump challenged Mr Khan to an IQ test, after the Mayor said the President's views on Islam were "ignorant".

Then, following the terrorist attack on London Bridge and Borough market in 2017, the President accused Mr Khan of "pathetic" behaviour.

Last July, Trump said Mr Khan had "done a very bad job on terrorism".

Mr Khan's comment comes a day after Trump on Saturday backed former Foreign Secretary Johnson to succeed outgoing Prime Minister Theresa May in an interview with the Sun newspaper, the Guardian reported.

In the interview, Trump also described Meghan Markle, the Duchess of Sussex, as "nasty" for calling him a "misogynist" in 2016.

In another interview in the Sunday Times he said he would want "to know" Labour Party leader Jeremy Corbyn before sharing American intelligence and suggested Brexit Party chief Nigel Farage negotiate with Brussels if the European Union (EU) failed to give Britain what it wants.

Meanwhile, at least 250,000 people are expected to protest in central London on Tuesday morning, on a route between Trafalgar Square and Parliament Square, when Trump meets May in Downing Street.