Sophia Money-Coutts: Kate Middleton's troubling new look

Up until now, you may have led a simple life, untroubled by the word "athleisure". Photo / Getty Images

Far be it from me to criticise the Duchess of Cambridge. She has hair shinier than a spaniel, looks heroically animated when she has to attend something boring, and the second that a new photo of her appears online, everyone in the Tatler office gathers around a computer and coos in approval as if gazing at a wrinkled newborn. In short, I am a fan.

However, she recently stepped out in an outfit that troubled me. You may have been alarmed too. She was in a shellsuit. Sort of. It was actually a pair of black and white tracksuit bottoms and a sporty, black zip-up top, paired with a pair of trainers so white that the old retinas winced a bit when you looked directly at them.

To be fair, it was an event at the UK's National Tennis Centre and therefore an occasion that called more for a tracksuit than a lacy Alice Temperley dress. And also the D of C is pregnant and has been feeling rotten, so I don't want to be too Lady Bracknell about a Royal stepping out in public wearing something comfortable instead of spindly heels.

But when I did some serious investigative work on the outfit by tapping "kate middleton tracksuit" into Google, I discovered that the trousers were £259 (NZD$491) "tuxedo track pants" and it's made me think about that dreadful word "athleisure".

To be fair, it was an event at the UK's National Tennis Centre and therefore an occasion that called more for a tracksuit than a lacy Alice Temperley dress. Photo / Getty Images

Up until now, you may have led a simple life, untroubled by this word, a clumsy portmanteau of athletics and leisure. But I'm going to ruin that for you by discussing this trend for wearing gym kit all day because, clearly, athleisure's influence is now so widespread it can't be too long before we see the Queen at Ascot in a pair of Lycra leggings and a sparkly top with a slogan that says "Yoga made me do it".

The fad for wearing sporty stuff has been with us for a while. Some point, inevitably, to America's influence, some to the 2012 Olympics. Whatever. It's here. It's convenient. It's being produced by big expensive brands (Valentino has just brought out a range of branded baseball hats and yoga mats), and less expensive, more mainstream ones (Beyoncé's "activewear" line for Topshop.)

Vogue has even issued athleisure rules, which include helpful tips such as making sure the fabric of your leggings is thick enough "so it's not see-through when you bend forward" and "you can't see cellulite through it". Apparently you also need to "accessorise appropriately" with "a pair of mirrored sunglasses" or "a shirt tied around the waist". And here was I tying items of clothing round my waist when I got too hot or wanted to disguise the size of my bottom!

So. Since athleisure's now being adopted by the very grandest of people, I thought I should issue a few rules of my own for those who may still be confused about when to wear sporty clothes and when to wear black tie, and they are as follows.

Do not attempt to do anything sporty in your athleisure. Largely because to really get that Duchess of Cambridge look you have to "invest in pieces" (Vogue again) which may mean you also have to remortgage, so you shouldn't do anything so common as to sweat in them. Givenchy's sold out of its classic black leggings for $1,175, I'm afraid, but Fendi has a lovely little tennis skirt for £530 ($1006).

And beware copying Kim Kardashian who went out for dinner in LA last week in a pair of grey tracksuit bottoms and a sports bra so small it looked more like an Alice band. This is wise and very personal advice since, last week when I myself went out for dinner - to the pub for a chicken and mushroom pie - I had to undo the top button of my jeans on the way home and, should I have been wearing a sports bra up top, I fear I may have been arrested for public indecency.