Mayor of London Sadiq Khan has compared Donald Trump's language to that of 'a 20th century fascist' and accused him of 'shamelessly' interfering in the Conservative party leadership race.

Ahead of the US President's three-day State visit to the UK, starting tomorrow, Khan also condemned the decision to roll-out the red carpet for the UK's closest ally.

It is the latest instalment in a long-running feud between the multimillionaire and Tooting-born Labour Mayor that dates back to 2016.

Trump failed to mention the London mayor in an interview with The Sun last week, ahead of his State visit, where he described Meghan Markle as 'nasty'.

Writing in the Observer, Khan said: 'President Trump is just one of the most egregious examples of a growing global threat.

Mayor of London Sadiq Khan, pictured last night at the Champions League final, has compared Donald Trump's language to that of 'a 20th century fascist' and accused him of 'shamelessly' interfering in the Conservative party leadership race

'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.

'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.'

The pair's feud was sparked in 2016 when Trump challenged Sadiq to an IQ test after the Mayor said his views on Islam were 'ignorant'.

The comments were made ahead of the US President's three-day State visit to the UK, which starts tomorrow

Speaking to the BBC shortly afterwards, Sadiq Khan said: 'Look it was Donald Trump that began this by saying his blanket ban on muslims would make an exception for me as Mayor of London.

'And the point I made with respect to Donald Trump is there's nothing exceptional about me. What about other Londoners who are business people, men and women, who happen to be muslim and want to do business in America?

'What about other Londoners who want to go and study in America but happen to be muslim?'

They clashed again in June 2017 when the US President called Khan's response to the London Bridge terror attack 'pathetic'.

Khan had previously warned Londoners that there would be an increased police presence on the streets and that there was 'no reason to be alarmed'.

Hundreds of thousands of protesters are expected in central London when Trump comes to meet Prime Minister Theresa May in Downing Street.

The 'Trump baby' blimp and a giant robot Trump sitting on a gold toilet are also likely to make an appearance.