The King’s state visit to America was an ‘absolute triumph’ which helped reset the strained relationship between President Donald Trump and Sir Keir Starmer, a senior Labour Minister has admitted.

Blair McDougall, Under-Secretary of State for Small Business and Economic Transformation, co-hosted the ‘Greater Together Los Angeles’ exposition - the largest transatlantic trade mission in history - alongside Lisa Nandy, Secretary of State for Culture, Media and Sport.

In an exclusive interview with The Mail on Sunday, McDougall said the King’s charm offensive when he met Trump in Washington DC during his four-day state visit at the end of last month was a diplomatic masterstroke: ‘Political ups and downs come and go. His Majesty’s state visit was an absolute triumph. He absolutely unlocked the US and renewed that deep relationship that we have.’

President Trump has made no secret of his distaste for the UK Prime Minister referring to Starmer as ‘weak and pathetic’ and condemning him for failing to allow US aircraft to use British bases for strikes against Iran.

But during the visit by the King and Queen he noticeably thawed with a source close to Trump telling the MoS: ‘It’s no secret the President has little time for Starmer. He thinks the guy’s an idiot but the president loves the Royal family and was blown away by Charles and Camilla.’

McDougall said he noticed a shift in mood within the government after the royal visit: ‘There was a real feeling across government. We were singing on the plane on the way over to Los Angeles and you could feel that optimism and confidence.

His Majesty unlocked that opportunity for us.’

A positively giddy Trump announced the removal of ten per cent tariffs on Scottish whisky at the end of the King’s visit.

McDougall said: ‘We managed to get preferential treatment and I think a lot of the analysis about why that is the case is because Donald Trump feels very warmly towards the UK. And why is that? It’s because of those deeper cultural relationships. The more hard-nosed economic things are built on that deep historical, cultural and almost familial relationship that we have. This mission is about trying to leverage that.

‘Every relationship has its ups and downs but like any family you come back together and I think that’s what we are seeing.’

Referring to the leadership contest which is tearing his party apart, McDougall said: ‘Whatever internal debates are going on within the Labour Party and whatever debates are going on within British society, one of the exciting things around the industrial strategy that I oversee is that we’ve built a consensus.

‘We’re very much getting on with the job and getting on with business and that is important in attracting investment to the UK because they (American investors) need to know that regardless of what happens in politics internally or externally this is a long-term plan that we’re committed to.

‘That’s what gives people that sense of confidence because we’ve signalled to the world very clearly that we’re opened for business.’

Referring to the King and Queen’s visit he added: ‘People are walking a little bit taller on the international stage as a result. It’s great. People are really happy and really excited.’

More than 230 British businesses were represented at the exposition which included appearances by celebrities like Britain’s Got Talent creator Simon Cowell and singer Leona Lewis as well as former England football boss Sir Gareth Southgate.

British business leaders such as Sir Lucian Grainge, Chairman of Universal Music Group and Dame Donna Langley, Chairwoman of Universal Pictures, joined the UK Ambassador Sir Christian Turner (who replaced Lord Peter Mandelson) and ex-Apple designer Sir Johnny Ive for a lavish reception at the Academy Museum of Motion Pictures.

VIPs included astronaut Major Tim Peake and the CEOs of British Airways and American Airlines.

Ambassador Turner said: ‘Britain is open for business. In an uncertain world the UK stands out as a safe harbour for capital and ideas.’

The US and UK enjoy $430 billion in annual trade with $1.7 trillion invested in each other’s economies. The US is the single biggest investor in the UK.