What do I wear under a kimono?

Coming from the UK, I am to wear a kimono at my son's wedding. His (Japanese) fiancee tells me one wears 'normal' underclothes. Should these be the basics, or would a petticoat or vest be appropriate?

If it's for a formal occasion I imagine the kimono to be the whole full set, so I assume you will have someone to dress you up? There are bras for kimono (that rather hugs your breasts instead of making the shape more conspicuous), and there are long underwear for kimono (called "juban").

I have to say that usually people don't wear what is "normal underclothes" to UK citizens. Perhaps the fiance is young and not familiar with traditional attire. Not many young women are.

When wearing a kimono, you are expected to wear a "hadajuban" and "koshimaki" directly on your naked skin (the "juban" comes over those). Traditionally, you don't wear panties, but nowadays most women do. A week ago I happened to be in a dressing room full of women, young and old, changing from Western clothes to kimono, and as far as I can tell, everyone took off their Western bras.

As mentioned, I'm sure you're having someone dress you up. Ask that person what you should bring yourself. Typically, they will tell you to bring your hadajuban, koshimaki and 2 or 3 koshi-himo (straps). Whatever they tell you, those items should all be readily available in reasonable prices at department stores in Japan.

The following is just for your reference.
http://www.japaneselifestyle.com.au/fashion/hadajuban_kimono.html

Thank you Uco, I did wonder if this is the case. The family have been both to the shrine & clothes hire shop & I had hoped any such instructions would have been forthcoming. I'll have to pursue the matter with my son.
It won't bother me if I have to strip off with everyone else, but my mother is also coming & she's become very self conscious of her aging body. Might be better not to warn her of the possibility, she'll only worry!

You won't "have to strip off with everyone else."

Please excuse me if my previous post was misleading. I just happened to be in a special situation (a backstage room, to be precise) but still we covered ourselves with the big kimono as we undressed ourselves, so that no one had to see our aging bodies!

But usually, you will be the only person in the room when you take off your bra to change into the hadajuban. Then typically the clerk will ask from the other side of the door or curtain to see if you're done, and if you are they will come in and see you all dressed in the Japanese undergarments as shown in the link I posted previously. They might adjust these undergarments a little, but professionals do it so well that they will never see any part of your body you don't show at a swimming pool.

Needless to say, the dressers are always females and these undergarments are as thick as T-shirts that you can't see through. But do be sure to take off your bra, because it will really get in your way as the big obi belt comes right under your breasts. In other words, the obi will secure your breasts therefore not requiring any bras.

Don't worry about it. It will be a splendid experience, I am sure.