The dialogue is so enchantingly sweet, it seems like it was plucked straight from a Disney film. On their nuptial day, the young prince and his bride are seen riding in an open carriage, sending waves to the cheering crowds.

‘Are you happy?’ she whispers to him.

‘It’s been the perfect day,’ he replies. ‘My face is hurting from smiling at everyone so much.’

She glances at him, coy and affectionate. ‘Tell me you love me,’ she says. ‘I love you always,’ he tells her earnestly.

That exchange was captured by cameras in 2012 as newlyweds William and Catherine left Westminster Abbey, on their way back to Buckingham Palace.

Experts appear to have decoded their words through careful analysis of the footage for a Channel 5 special airing tonight, titled Lip-Reading The Royals: The Secret Conversations.

Compare this to the interaction between Charles, then Prince of Wales, and his bride Camilla, on their own wedding day at Windsor in 2005. Upon exiting St George’s Chapel, they faced a strong gust of wind, nearly dislodging the Philip Treacy fascinator from her head. A halo of golden feathers, it danced wildly.

‘I wish I hadn’t had feathers,’ the bride fretted. ‘They look awful,’ Charles appeared to reply, with callous honesty that in other families might have earned him a kick to the shin, or even an immediate annulment.

His mother the Queen didn’t help matters by voicing her agreement. The words are not audible but her lips spell out unmistakably: ‘I did forewarn her not to wear feathers.’

It’s an embarrassingly candid moment, one of many in a show that barely skims the surface of the available material. In an era when every royal appearance might be filmed from the sidelines by a member of the public, in high-definition video on a smartphone, the potential for gaffes is almost infinite.

So far, mostly, the worst blushes have been spared. There is nothing in this 90-minute documentary featuring Prince Andrew, a man who apparently has almost no conscious control of what blurts out of his mouth. Nor do we see anything of his father, Prince Philip, whose rudeness was legendary.

But innocuous though most of it is, this cheaply-made programme marks a turning point for the Royal Family – putting them on notice that from now on, every murmured aside is potentially damning.

Even the most innocent comment will be captured and analysed. Every royal will now know that no indiscreet remark is safe. If they want to keep anything private, they will have to cover their mouths like footballers discussing tactics on the pitch – which looks both suspicious and inelegant. The only safe solution will be to say nothing. Queen Elizabeth II famously lived by the motto, ‘Never complain, never explain’. To which her descendants will have to add, ‘Never say anything at all.’

What’s so fascinating about the evidence from lip-readers in this show is how much the deciphered remarks match our preconceptions. It’s no surprise to discover William and Kate were so much in love on their wedding day – nor that Charles and his first wife, Diana Spencer, were so awkward on theirs.

As the Prince of Wales stood beside his bride-to-be in St Paul’s, he muttered something to her. The lip-readers caught it: ‘You will be fine,’ he told her.

Those are not the words a 19-year-old longs to hear from her bridegroom. But Di tried to make light of it, answering him in a singsong voice, ‘As will you!’

A decade later, sadly, they were barely on speaking terms – reduced to small talk when they had to acknowledge each other.

On a tour of South Korea, following revelations that their marriage was on the rocks, they were filmed in the back of a limousine. Diana was pressed against the side window, shrinking as far away from her husband as she could. ‘I think South Korea is quite interesting, don’t you?’ he seems to suggest.

And without looking at him, she retorts, ‘Yep!’

One exchange between Prince Harry and his wife, Meghan, at a Remembrance service in 2018, also epitomises the conversations we might imagine they have in secret. It’s not clear what they’re discussing, but they’re definitely plotting something.

‘Take advantage of this situation,’ she urges him, according to the lip reader’s analysis. Harry gapes a bit. ‘Today?’ he asks.

‘Do it tonight,’ she orders.

The prince looks more uncomfortable still. ‘You do realise that this is the end?’ he says.

If that exchange occurred in an episode of The Crown, it would seem too stereotypical – Meghan scheming like Cruella de Vil, Harry vacillating like a minor character in a Jeeves and Wooster story. But the wretched aftermath of their breakaway from the rest of the Royal Family is summed up by his complaints on the steps of Westminster Abbey, as the congregation files out at the end of the Coronation ceremony in 2023.

Harry was on his own, since Meghan opted to remain in California. Amid the crowd of relatives and former friends, he was being comprehensively ignored.

Sidling over to Princess Eugenie, one of the few people still willing to talk to him, he appeared to moan, ‘I’ve spent five hours with my dad in his office and not a friendly text from either.’

Who hasn’t texted him? Charles and Camilla? William and Kate? The answer, of course, is all of them. And then he bleats again, ‘Why hasn’t anybody contacted me?’ Eugenie gives him a sympathetic but wary smile.

No one wants to end up in his next memoir. Wariness is something all the royals will have to practise from now on. It’s a learned skill, something the Princess of Wales hadn’t fully mastered when she was still Kate Middleton.

At Sandhurst in 2009, she and two girlfriends were admiring William in his dress uniform. ‘I love the uniform,’ she gasped. ‘It’s so sexy.’ Her friends squealed with laughter. No harm done there. But the capacity for catastrophe will always be a slip of the tongue away, now that the lip-readers are watching. The King is only too aware of how easily things can go wrong. On the balcony of Buckingham Palace on his Coronation Day, he struggled to keep his head perfectly still. ‘Don’t get too close,’ he warned Camilla. ‘My crown might topple over.’

As he knows too well, the crown is always one wobble away from disaster.