Oh dear. It’s all a bit awkward, isn’t it? Princesses Beatrice and Eugenie, dressed to the nines in expensive designer outfits that don’t quite add up, appearing to look daggers at the nape of Kate’s swan-like neck as she glides effortlessly ahead, a symphony in simple nudes and pale yellow.

Poor loves. I know exactly how they feel. I’ve always, always been the frumpy one in any given photograph. The one whose arm is at an odd angle, whose grin is just that little bit too wide, whose leg is askew.

Even when I’ve made the utmost effort - as Beatrice and Eugenie clearly have here - I end up looking completely hideous.

Beatrice and Eugenie are really pretty. Lovely hair, nice eyes, good figures. It’s just that next to a woman as genetically blessed as Catherine, they don’t stand a chance

And yet I am not a total monster. I can, I am told, look quite nice on occasion. As can the York sisters - in fact, if you see them in the flesh, Beatrice and Eugenie are really pretty. Lovely hair, nice eyes, good figures.

It’s just that next to a woman as genetically blessed as Catherine, they don’t stand a chance.

It’s not just the clothes; it’s the proportions. How can anyone compete with an ankle as well-turned as Catherine’s? And those elegant wrists, that narrow frame. They are dobbins to her racehorse. Nothing they wear or do will ever change that. They are simply different breeds.

Catherine, with her willowy frame, perfect proportions and undeniable beauty, belongs to the same species as Georgia May Jagger and Rosie Huntington-Whiteley. There are variations within the genus, but essentially they’re the beauty master race.

Catherine, with her willowy frame, perfect proportions and undeniable beauty, belongs to the same species as Georgia May Jagger and Rosie Huntington-Whiteley

It doesn’t matter how badly they behave, how much they smoke or drink or sit in the sun - they still look ten times better than everyone else. (As it happens, Catherine leads a saintly existence, which probably doesn’t endear her to the sisters who, it’s well known, like to have a bit of fun now and again.)

But at least Beatrice and Eugenie don’t have to put up with it from their own mother, as I do.

My mother, who is 20 years older than me, still looks better than me in most family photographs. And it’s not as though she’s trying, believe you me. It’s just that she has a natural elegance and beauty that I could never even hope to emulate.

'My mother, who is 20 years older than me, still looks better than me in most family photographs', says Sarah Vine

Irritatingly, my brother has it, too. I come from the other end of the gene pool. The short, stocky Welsh end. ‘Built like a brick sh** house, we Bynons are,’ my grandmother used to say cheerfully, her sleeves rolled up to show off her muscly forearms as she gutted fish or performed some other suitably unglamorous task.

I live in a world where every day looking good on camera counts for more and more - and yet, like Beatrice and Eugenie, it’s the one thing I simply can’t achieve

If I’d been born in a less photo-centric age, it wouldn’t have mattered. In fact, it would probably have played in my favour. No chance of getting kidnapped by passing barons and blowing the housekeeping on a ransom.

Plus, I probably would have been highly sought after by gentlemen looking for the sort of all-weather wife who can dig a ditch with one arm and hold a baby in the other.

Sadly, it was not to be. I live in a world where every day looking good on camera counts for more and more - and yet, like Beatrice and Eugenie, it’s the one thing I simply can’t achieve.

You’ve either got it or you haven’t, and I really haven’t.

Most of the time, of course, it’s neither here nor there. As a writer I can hide behind my words and no one need be any the wiser. But occasionally I must change out of my tracksuit bottoms and venture out into the world, where the threat of being ambushed by a photo lens is ever present.

That is why I have learnt, over the years, to stay well out of shot when travelling with Catherine-like friends.

You never want to be photographed standing next to the Prime Minister’s wife, Samantha Cameron (pictured with Sarah at the recent State Opening of Parliament)

The Prime Minister’s wife, Samantha Cameron, for example: you never want to be photographed standing next to her.

Nor, for that matter, the wife of the Chancellor, Frances Osborne, who has a waist like a whippet.

Then there’s my glamorous friend who used to be an actress and her friend, actress Helena Bonham Carter.

I once ended up sandwiched between them at a charity do and it looked as if they were propping up an overweight, alcoholic transvestite.

Joan Collins is so photogenic it’s criminal. It’s not just that these people are better looking; they also know how to work the camera

Worse of all is probably Joan Collins: that woman is so photogenic it’s criminal. Even Christopher Biggins looks better in photographs than me.

It’s not just that these people are better looking; they also know how to work the camera.

Because I’m already dreading the outcome, I tend to shrink from the lens. I make wonky shapes with my mouth, my clothes flap open at such unflattering angles, my eyes go all googly.

A picture of me at the recent State Opening of Parliament is a case in point: it shows me looking like a very grumpy dowager duchess, peering through a pair of binoculars at Her Majesty.

I thought I looked rather foxy and fabulous in my swanky jacket and fascinator. Photographic evidence indicates the opposite. I’d forgotten to turn up the edges of my mouth, my jacket was doing something odd around the armholes - and the angle of the shot gave me a double chin and a turkey neck.

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The same is true of Beatrice and Eugenie. They’re too busy focusing on how annoyingly fabulous Catherine looks to remember to compose their faces.

The Duchess, by contrast, knows we’re all gawping at her and she’s making sure everything’s just as it should be.

I have learnt to walk at least ten paces behind anyone attractive in a potential paparazzi situation.

Failing that, find a nice, roly-poly uncle-like figure (the more double chins the better) to stand next to.

The other option, of course, is a hat. With a very large brim.

One crumb of consolation: Beatrice and Eugenie may not look as soignee as they might have hoped in this picture.