RICHARD KAY: What was it about Trump’s behaviour that might have prompted the Queen to make such a comment after years of discretion?

For 70 years, no figure in public life had greater patience, nor had his tolerance been tested more often. But despite many provocations – and considerable temptations – the late Queen Elizabeth endured them all with a firm omerta.

It served her well during her long and successful reign. So much so that – towards the end of her life – we know as little about what the Queen thought about things as we did when she first took the throne.

Her views on party politics, on celebrity, on the rich and famous she met over the decades were all taboo to us, her subjects. But every now and then there was a catch and the Queen’s views emerged from beneath that royal reserve.

And when that happened, it was usually accompanied by fireworks. Her fury, for example, when David Cameron broke official protocol not once but twice by revealing how he had asked her to intervene in the Scottish referendum and later revealed how she had “cut the line” when he broke the news that the vote to break up the Union had been lost.

She also said she did not like Tony Blair revealing his private meetings with her and that she felt he showed a “certain hubris”.

And there was royal dissatisfaction over his claim that Princess Anne could be “extremely rude”.

But there was one rule she was particularly strict about: no visiting head of state, no matter how horrible, would ever hear a word of criticism from her lips. Of course, things occasionally got out.

Take, for example, the appalling behavior of Chinese President Xi’s advisers, who made ridiculous demands on Buckingham Palace staff, including demanding that the science of feng shui – said to optimize the flow of energy in a room – be used in preparing the accommodation for the Chinese leader.

US President Donald Trump, Queen Elizabeth II and Melania Trump at Buckingham Palace

Donald Trump and the Queen at Windsor Castle in July 2018

She let slip, and a TV microphone picked it up, that the Chinese had been ‘very rude’ to our ambassador. It caused a storm, with the Chinese state media lashing out at the ‘reckless gossips and barbarians’ of the British media.

The then Foreign Secretary, Philip Hammond, had to make a groveling apology.

That is why the comments in Craig Brown’s new book about Donald Trump, the then US president – ​​our closest and most important ally – seem so unusual.

What was it about Trump that could prompt the Queen to abandon the diplomatic pleasantries for which she was so famous? He did not leave the palace’s Belgian Suite in the state that other visiting political leaders had; nor did he whip out a laptop during tea at Windsor Castle, as that Chinese delegation infamously did.

Certainly, the memory of the day Trump arrived at the castle and inspected the Queen’s guard still lives on. The president was so clearly in awe of his host, despite towering over her.

But according to Craig Brown, within weeks of that brief visit, the queen was so irritated that she not only complained about Trump but also passed judgment on his marriage.

Brown quotes a source as saying she felt the 45th US president was poorly behaved because of the way he looked over her shoulder, as if he was looking for someone more interesting.

It seems extraordinary that she would admit something like that, even to a very good friend.

And then there’s the shocking comment she’s said to have made about the state of the president’s marriage, speculating that Mrs. Trump “must have some sort of arrangement” because why else would she have stayed married?

What makes these comments so incredible is that she should have ventured them in the first place. We are used to Prince Harry pouring out his innermost thoughts – but not the late Queen. Of course, it should be noted that the Queen did not expect these views to see the light of day.

Even a cursory understanding of her life would show that such an intervention is highly unusual, nay, virtually unheard of. What, one wonders, was it in Trump’s behavior that made her make a comment she never made about any of the other 13 sitting American presidents she met in her life?

I’ve heard other members of the royal family make similar comments about boring types who are constantly scanning the room for someone more interesting to talk to, but never the Queen.

On the other hand, she always enjoyed those meetings – whether in the White House or here in Britain – with the leader of the free world. Some she loved: President Reagan, with whom she rode, and President Roosevelt, who was so close to her father, King George VI, that he arranged for him to be godfather to Prince Michael of Kent.

Moreover, the Queen is always well informed about the visiting heads of state and reads gossip about their private lives.

That’s what makes her decision to use decades of discretion to express her opinions about Trump so bizarre.

He will surely answer. In his usual modest way, he boasted of their meeting that ‘there are people who say they have never seen the Queen have a better time, a more animated time’.

Maybe he thinks differently now.