US President Donald Trump has hailed the special relationship between his country and Britain as he paid a gushing tribute to King Charles during his historic second state visit, calling it one of the highest honours of his life.

It was a day of unprecedented pomp for a foreign leader. Trump and his wife Melania were treated to the full array of British pageantry.

Then, the president sang the praises of his nation's close ally.

"The bond of kinship and identity between America and the United Kingdom is priceless and eternal. It's irreplaceable and unbreakable," Trump said in a speech during a lavish banquet at Windsor Castle, family home to British monarchs for almost 1000 years.

Referring to the so-called special relationship between the two nations, Trump said:

"Seen from American eyes, the word special does not begin to do it justice."

Trump's speech will be music to the ears of British Prime Minister Keir Starmer.

He proffered a state visit to win favour with Trump the well-known anglophile and overt royal fan, shortly after the US president returned to office in January.

Starmer hopes the trip will aid his government as it seeks to deepen economic ties, secure billions of dollars of investment, ease tariffs and allow him to press the president on Ukraine and Israel.

Trump basks in a display of regal splendour. – AP

Britain rolled out the royal red carpet, giving Trump the largest military ceremonial welcome for a state visit in living memory. Trump made little secret of his delight at being not just the first U.S. leader, but the first elected politician to be invited for two state visits.

"This is truly one of the highest honours of my life," he said.

Over the course of the visit, Britain is hoping to convert Trump's affection for Britain – his mother came from Scotland – and admiration for the royals into concrete actions.

Companies including Microsoft, Nvidia, Google and OpenAI have already pledged £31 billion in British investments over the next few years, in AI, quantum computing and civil nuclear energy.

Starmer also wants further progress on trade, after Britain secured the first deal with Trump to lower some tariffs. Talks may touch on remaining levies on steel, whisky and salmon.

"The United Kingdom was your partner in the first trade deal of your administration, Mr President, bringing jobs and growth to both our countries," the king said in his speech.

"And no doubt we can go even further as we build this new era of our partnership."

'We are the closest of kin': King Charles toasts Trump at Windsor banquet. – Reuters

But while Starmer is banking on the royals to help cajole the president, pitfalls remain.

Polls show Trump is widely unpopular in Britain and Starmer, faced with plummeting poll ratings of his own and economic woes, will need to show his royal trump card can reap benefits.

While there were many Trump supporters in crowds at Windsor in London, several thousand people marched to protest against the state visit.

"I quite simply dislike everything that Trump and his administration represent around the globe," retiree Bryan Murray said.

Amongst the guests at the banquet was Rupert Murdoch, whose Wall Street Journal publication the president is currently suing in a $10 billion defamation case.

While there was a massive security operation in place in Windsor, police said four people had been arrested on Tuesday after protest images were projected onto one of the castle's towers. Trump was not there at the time.

The Times newspaper has also reported that Britain would announce it was recognising a Palestinian state after Trump had departed.

The US opposes that move, which could prove a thorny issue in their talks.

Trumps and UK King and Queen watch military parade and flypast at Windsor Castle. – AP

While the focus tomorrow will turn to geopolitics and trade when Starmer hosts Trump at his Chequers country residence, today was all about ceremony.

Trump and Melania joined Charles, his wife Queen Camilla and other royals and dignitaries for a carriage procession, with the route lined by 1300 British service personnel.

Later, the Trumps viewed historical items from the Royal Collection relating to the US, and then visited St George's Chapel, the final resting place of Queen Elizabeth, who hosted Trump for his first state visit in 2019, to lay a wreath on her tomb. She died in September 2022.

There was a further military parade and a flypast by Britain's Red Arrows aerobatics team, but poor weather meant British and US F-35 military jets – a symbol of bilateral defence collaboration – could not join.

The Trumps also found time for a private meeting with the king's elder son Prince William and the heir's wife Kate, which was described by the prince's spokesperson as "warm and friendly".

Trump later praised "beautiful" Kate and said William was "going to have unbelievable success in the future".

As for Charles, the 76-year-old monarch, he was a "very, very special man", the president said.

Trump inspects guard of honour as state visit to UK begins. – Reuters