America and Britain have had close operational ties on defence, security and intelligence in the decades since Winston Churchill coined the phrase “Special Relationship” in 1946, just after a devastating world war left Britain weaker and America emboldened.

The US is Britain’s biggest trading partner outside the EU and the countries invest more than a trillion dollars in each other every year – the most of any two nations on earth.

As Britain leaves the EU, its Government needs America’s trading partnership more than ever – but its people revile Donald Trump. Yesterday, UK politicians attacked him after he retweeted three anti-Islam videos by the far right Britain First group, and then attacked Theresa May for criticising him for it.

Sir Christopher Meyer, who served from 1997 to 2003, said it could cause “gross offence” to the US.

“To kick away the legs to our relationship with the US, just at a time when our negotiations with Europe will be moving to a crucial stage, would be folly, absolute folly,” Meyer said, adding it could offend Americans who oppose Trump but would see it as an insult to the presidency.

Peter Jay, who was ambassador from 1977 to 1979, said he could imagine “few things more damaging” than withdrawing the invitation.

But he called Trump and May’s relationship “the most spectacularly uncomfortable” he had ever seen between a prime minister and president.

“The idea of the relationship in the form of a special bond between the two heads of government is and is likely to remain something of a fantasy under the present leadership in Washington,” he told HuffPost.

“Mrs May may have to face up to the fact that that horse has bolted.”

Jay remembers when he was ambassador, Prime Minister James Callaghan was at a G7 summit in Guadeloupe in 1979, where the leaders stayed in thatched cottages, and “was able to stroll across to Jimmy’s cottage” to have a “man-to-man chat” about Britain’s nuclear weaponry, for which the Americans provided the hardware.

“There was something special about those two guys. They had a relationship because of previous meetings. It was very important,” Jay said.

But he added Britain and America’s working relationship on matters like defence and intelligence was too ingrained to be irretrievably hurt by an erratic president. He pointed out presidents and PMs have clashed before and it endured.

When Churchill coined the phrase Special Relationship, Britain and America were arguing over a post-Second World War American loan. Years later, Lyndon Johnson resented Harold Wilson for not sending British soldiers to Vietnam.

Even Margaret Thatcher, who was practically political soulmates with Ronald Reagan, berated him for America’s 1983 invasion of former British colony Grenada.

Meyer sees Trump as part of the “ups and downs” of Anglo-American relations, which are “marked more by volatility than stability”.

“In the past, we’ve always got over these spats pretty quickly because intrinsically it’s a very, very strong relationship. The difference about this one is it’s the most public because Trump uses Twitter.

“How many outrages has Trump committed since he became president? The list is endless. We’ve forgotten the old ones because new ones keep piling in.”

He said of Trump’s latest attack: “Life goes on.”

Both Jay and Meyer view Britain and America’s working relationship as so embedded that Trump’s erratic tweeting doesn’t gravely threaten it. As an example, Britain’s Royal Navy is set to buy American jets to put on its new aircraft carriers.

“The very large machinery in defence. Intelligence. Economic and the commercial sphere carries on regardless. An awful lot is happening despite [Trump],” Meyer added.

Dr Karin von Hippel, director-general of British international relations think-tank RUSI and a former counter-terrorism advisor at the US State Department, said American-UK co-operation in areas such as ambassadors working together in third countries and security services sharing intelligence was continuing unaffected.

“I don’t see his tweets or his irresponsible behaviour or his outbursts damaging the relationship,” she said.

“Most Americans who work on this stuff think Trump is being ridiculous. I would imagine most Americans who are counterparts to Britain in a variety of different jobs are embarrassed about what Trump is doing.

“I suspect they’re rolling their eyes as much as everyone else globally seems to be.”

She noted that even at the highest echelons of American Government – people like Secretary of State Rex Tillerson and UN ambassador Nikki Haley – have had to walk back or contradict Trump’s public remarks. 

She said: “The responsible adults in the room are trying to maintain America’s standing. People are getting more and more immune [to Trump].”

Meyer says of those around Trump: “They have learned to work with him and around him and we must do the same.”

He compared the US-UK relationship to an iceberg, most of which is hidden beneath the surface.

“The stuff under the surface is the underpinning of everything. I still believe the US is our biggest ally and partner,” he said.

But the phrase “Special Relationship” is a “sentimalised oratorical device that doesn’t reflect reality,” he said.

He added: “We have to get out of the notion we have a relationship with the US that is different in kind to any other relationship. There’s a massive and still growing critical mass of interests which will grow, whoever is president and whoever is prime minister.”

Jay said he “always groaned inwardly” when he heard “Special Relationship”.

He said: “It’s always been unhelpful to go on about it. I’ve also said it’s better practised than talked about.”

For now, he added, the prospect of a close personal relationship between Trump and May is “on ice”.

“Mr Trump won’t be president of the United States forever,” Jay said. “And we may have to take comfort from that thought.”