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 1,000 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.

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 US leader, but the first elected politician to be invited for two state visits.

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 pounds (US$42 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." 

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.