RICHARD KAY: What prompted Queen to shade ‘very rude’ Trump after years of discretion?

For 70 years no figure in public life had a greater capacity for patience and nor had that forbearance been tested more often.

Yet despite many provocations – and considerable temptation – the late Queen Elizabeth weathered them all with a sturdy omerta.

It stood her in great stead throughout her long and successful reign.

So much so, that – by the end of her life – we knew as little of what the Queen thought about things as we had when she came to the throne.

Her views on party politics, on celebrities, on the rich and famous that she met over the decades, all were to us, her subjects, off limits.

But just occasionally there was a fly in the ointment and the Queen’s view would emerge from behind that regal reticence.

And when it did, it was usually accompanied by fireworks.

Her anger, for example, when David Cameron not once but twice breached official protocol by revealing how he had asked her to intervene in the Scottish referendum and then later disclosed how she had “purred down the line” when he broke the news that the vote to break up the Union had been lost.

She also let it be known that she didn’t like Tony Blair revealing his private encounters with her and whom she accused of exhibiting a “certain hauteur”.

And there was royal displeasure at his claim that Princess Anne could be “stunningly rude”.

But there was one rule she was especially firm about: no visiting head of state, however ghastly, would ever hear a word of criticism from her lips.

Of course, from time to time things did get out.

The appalling behaviour of President Xi of China’s aides, for example, who imposed ludicrous demands on Buckingham Palace staff, including insisting the science of feng shui – which is said to be able to optimise the flow of energy in a room – was used when preparing accommodation for the Chinese leader.

She let slip, and a TV microphone picked it up, that the Chinese had been “very rude” to our ambassador.

It provoked a storm, with China’s state media lashing out at the “reckless gossip fiends and barbarians’ of the British media.

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

Which is why the comments reported in Craig Brown’s new book about Donald Trump, the then President of the US – our closest and most important ally – seem so out of character.

What was it about Trump that could have provoked the Queen to abandon the diplomatic niceties for which she was so famous.

He did not leave the Palace’s Belgian Suite in the sort of state that some other visiting political leaders had done; nor did he get 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 lingers in the memory.

The President was so clearly in awe of his host despite towering over her.

Yet within a few short weeks of that brief visit, the Queen, according to Craig Brown, was exercised enough to not just complain about Trump but also to pass judgment on his marriage.

Brown quotes a source saying how ill-mannered she found the US’s 45th President because of the way he looked over her shoulder, as though seeking out someone more interesting.

It seems extraordinary that she would have vouchsafed that, even to a very close friend.

Then there is the shocking remark she is said to have made about the state of the President’s marriage, speculating that Mrs Trump “must have some sort of arrangement” or why else would she have remained married?

What makes these remarks 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 pointed out that the Queen did not expect these views to see the light of day.

Even a cursory understanding of her life would show such an intervention to be highly unusual, nay, virtually unheard of.

What, one wonders, was it about Trump’s behaviour that saw her make an observation she never did about any of the other 13 sitting US presidents she met in her lifetime?

I have heard other members of the royal family make such a remark about bores who they meet who constantly scan the room for someone more interesting to talk to, but never the Queen.

On the other hand, she always enjoyed those encounters — either at the White House or in Britain — with the leader of the free world.

Some she was fond of: 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 become godfather to Prince Michael of Kent.

It is also the case that the Queen is always well informed about visiting heads of state, which includes perusing the gossip on their private lives.

Which makes her decision to abandon decades of discretion to vent her views on Trump so bizarre.

Surely he will respond.

In his usually modest way, he boastfully said of their encounter that “there are those that say they have never seen the Queen have a better time, a more animated time”.

He might have a different take on it now.