The late Queen might have used it, Presidents Obama and Trump used it and the King liked to use it back in his days as Prince Charles.

However, do not expect him to wax lyrical about the 'special relationship' this week.

A senior government source has told the Daily Mail that British diplomats now prefer to use terms such as 'enduring relationship' or 'extraordinary alliance' when describing UK/US links, rather than the famous phrase first coined by Winston Churchill 80 years ago.

This decision predates the current administration, we are told, and has nothing to do with President Trump's recent potshots at Sir Keir Starmer.

Rather, it is seen as just not terribly British – a tad arrogant, perhaps – to boast about a 'special relationship' with the most powerful nation on earth. Also, it annoys other allies.

For all that, the relationship could be about to turn even more 'special' following the Daily Mail's discovery that President Trump and the King are related.

We know that the avowedly royalist President Trump already regards the King as 'a great guy' and 'a fighter' – he told me so himself recently – but his affection for the monarch and the Royal Family may now become even more pronounced when he learns that he and the King share descent from the same Scottish nobleman.

Detailed research on behalf of the Daily Mail has shown they have a common ancestor in the form of the 3rd Earl of Lennox, a great-grandson of King James II of Scotland, which means that Donald Trump and Charles III are 15th cousins. Their shared forebear met an unfortunate end, however.

Lord Lennox was involved in a vicious power struggle for control over the infant King James V and was eventually defeated at the Battle of Linlithgow Bridge in 1526.

After being taken prisoner, he was murdered by a rival laird known as the Bastard of Arran. His son, the 4th Earl of Lennox, went on to produce Lord Darnley, husband of Mary, Queen of Scots. Their son became King James I of England, from whom the Stuarts and, eventually, the House of Windsor would all descend.

Lord Lennox had several children, including a daughter, Lady Helen, who married the 11th Earl of Sutherland. Their son, the 12th Earl, survived attempted murder by poisoning (which killed the 11th Earl) and produced a daughter, Lady Jane.

She went on to marry a chief of the Clan Mackay and their son, the much-married Donald Mackay, was created 1st Lord Reay – who also happened to be a staunch supporter of another King Charles.

From this Donald, through generations of Mackays and then of MacLeods (including another Donald), the line comes down to Mary Anne MacLeod of Lewis, who emigrated to the USA in 1930. There she married property developer, Fred Trump, and had five children. Donald Trump, born in 1946, was the fourth.

The President has previously spoken of his mother being 'a big fan' of Queen Elizabeth II, and he was of much the same mind.

As he explained to me while I was writing my biography of the late Queen: 'I had a really good relationship with her. She was unbelievable. I liked her and she liked me.'

He'd been particularly impressed by the Queen's refusal to name her favourite president, despite his best efforts to coax a name out of her at their first meeting in 2018.

That meeting had gone so well that what was supposed to be a short courtesy call ran on for the best part of an hour and was followed by a full state visit the following year – making President Trump the last state visitor of her reign.

He was equally thrilled by last year's state visit to Windsor at the invitation of the King. 'He's fantastic and he has fought very hard. He's a fighter,' Mr Trump told me. 'We're close. I have a really good relationship with him. Let me just give you the bottom line. He's a great guy and he's grown so much in the last ten years and especially over the last couple of years as King. His fight has shown that.'

All of which means that the President will be delighted to discover a direct lineal connection with the House of Windsor.

Online genealogists have previously struggled to ascertain a clear line of Mr Trump's ancestry due to confusion and gaps in ancient Scottish record-keeping.

However, veteran genealogist Robert Barrett, former research editor at the Daily Mail, has studied multiple land records, church records and Scottish peerage documents to piece together this intriguing research. For example, Mr Trump's descent from Donald Mackay is confused by his daughter's first marriage to a Macdonald, after which she married a clergyman called Hugh Munro, and the Trump line hails from there.

This earlier Donald/King Charles relationship was certainly 'special'.Donald Mackay was a staunch royalist who fought for King Charles I in the English Civil War, was imprisoned for a period and ended up in exile in Denmark, where he died.

He was also a man who liked an argument and was briefly interned in the Tower of London by the King in order to prevent him taking part in a duel.

President Trump may be equally pleased to learn that his pedigree also links him to the royal houses of Norway, Sweden and also Denmark. This casts a fresh perspective on his plans to annexe Danish-controlled Greenland from King Frederik X, since both men are descended from Denmark's King Christian I.

Whether this, in turn, makes the President more proprietorial or diplomatic on the Greenland issue remains to be seen.

Yesterday, Donald and Melania Trump welcomed the King and Queen to the White House informally, with a cup of tea and a look at the presidential bee hives. The formal ceremonial welcome takes place today and President Trump is planning to surprise his royal guests with an extra special event.

Charles and Camilla concluded yesterday's programme with a garden party for 600 British and American guests in the grounds of the famous Lutyens-designed British Embassy in Washington. The summons to enjoy scones, clotted cream and tea with the monarch has been one of the hottest invitations in Washington this year.

It all points to a relationship which continues to remain 'special', whatever the diplomats say. However, Mr Trump's new-found royal ancestry does present him with one problem. He's now related to George III, the 'tyrant King' whose expulsion from these shores in 1776 is precisely why the USA is celebrating its 250th birthday this year.