Cleavage heat rash/itch...

You need an anti fungal such as Canestan.

It's the same kind of issue as athlete's foot.

What MissA said, if you can find Daktarin Dry Spray in-line or in you pharmacy it is really effective.

Nappy liners are good for... ahem... moist areas.

Also, in very hot weather, leave the bra off!

It is similar to athlete's foot, dhobi itch and and jock itch (which presumably affects the jock-strap area) Treat it like athlete's foot, remembering to keep using the anti-fungal cream for 5 days after it goes away. Also while putting antiperspirant under your arms, add some under and between your boobs. Anthistan helps with the itch. Wash your hands after applying the fungal cream, or you'll transfer it somewhere else.

lemongrove Without the bra, the boobs lie even more heavily on the skin of the chest. Cotton bras will absorb more sweat than manmade fibres.

You have a “heavy bust” and wear crop tops?! What’s your secret?

Going without a bra if you have a heavy bust makes the sweating even worse in hot weather.

Lanacane medicated powder sprinkled between and under the breasts day and night works for me.

It sounds very much like thrush which is common in the folds beneath large breasts. I agree with Elegran, going without a bra would make it worse. If you are a G cup, I would suggest a really well fitting bra with maximum support to lift your breasts and help to keep the area of 'fold' to a minimum. And yes, I too have big boobs and I do sympathise.

I’m also struggling with this and have started wearing sports bras in this weather and dousing them in talc and it seems to have done the trick for me. The only problem is that the sports bras aren’t quite as supportive as a normal bra, but I’m willing to have squishy weird shapes boobs for a few weeks rather than have to wring my tops out every 20 minutes!?

Prickly Heat Powder works.

I had this years ago. Although I have lived abroad it was in this country, oddly enough, and that is what the chemist recommended and it worked.
Ask the chemist, don't just buy online.

I find Boots anti fungal cream works for me. Just ask over the counter.

I'm a G cup, too, but have never had that problem. My bras are all wired, and all fit properly (I pay £40+ each for a bra).

My friend is the same size as me, wears bras that don't fit, and she has heat rashes. She uses Sudocremat night, then washes it off in the morning.

Apologies if the ‘leave the bra off’ advice was wrong....it works for me, but then I am not a large lady.?

I used to have this problem. I am an H cup and wear well fitted underwired bras.

As pp said, Lanacane medicated powder is great but I haven’t been able to get it for a couple of years. There is another medicated powder which is excellent. It’s Medipure Body Powder.
I got it from Amazon in packs of 6 for around £12. Much cheaper than Lanacane. And now see it in Home Bargains for about £1, maybe a little more. Didn’t check the price properly as I have such a stock of it. The container is blue and yellow.

It’s years since I had the sore under booby.
At the last time I used Canestan and the medicated powder. Once cleared in a couple of days I have continued to use this powder daily. Never had a problem since.
Honestly I suggest you try this.

Try good old Boracic powder daily in the affected areas after showering or bathing. If this doesn't work then try the antfungal creams.