Does the camellia get the early morning sun when there might still be frost on the buds? This is the usual cause of the flowers dropping. Also if you have had the shrub in the same tub for 20 years have you been feeding with ericaceous feed? 20 years is a long time in the same container and they prefer acidic soil if at all possible.

West facing is a better aspect to avoid early morning sun as MG says. I assumed your plant was in the ground rather than a tub? Try using an acid mulch (even rotted lawn mowings are a bit acid, and so are coffee grounds, but peat is better unless you disapprove of using it. Also feed with an ericaceous fee. But if that were the trouble I would have thought the leaves would have shown signs of yellowing. It might be worth throwing some fleece over it at night at flowering time to shade it, to see if it is the early sun causing the trouble. If you protect part of the plant and not the rest it will be easy to see if it makes any difference, though it's probably too late this year unless you still have lots of unopened buds.

In my experience, rapid browning or yellowing of flowers on Camellia is much more noticeable, and happens much more quickly, on white and yellow flowered varieties - pink is only slightly better, the best is red. If this happens every year, and always has done, it's just the nature of the beast - I binned my beautiful double yellow flowered one, drove me mad because of this problem.
The only other possibility is it was flowering, or had started to flower, when the weather was warm, and when the temperature fell, the flowers were burned by the cold - but if this always happens, my first explanation is the most likely. Certainly, with healthy, green and glossy leaves, the plant is not suffering from chlorosis or a shortage of nutrients. However, you say yours is in a south facing situation, which means it gets full sun when we have sun - that will cause the flowers to brown more quickly too, at least on pale coloured varieties.

Thank's to all, yet not convinced of any advice! The Camillia is planted in a south facing border sheltered by
an adjacent plum tree, it is in very healthy condition throughout the year, has abundance of double white blooms which turn brown & fall with-in 24 hours? yes, it
does have a little sun this is low in a morning, then sheltered at full sun! It may need a feed? but this would reflect upon the whole shrub if in need of a feed?

I would add that this problem occurs every year!

Any more solutions?

Many thank's John A Sutton.

I would refer you back to my answer, where I do actually say your plant appears to be very healthy, and that this is a problem with the variety rather than situation or disease, particularly now you have confirmed it is shaded from sun during the day.
Although its interesting you mention early morning sun - this is one thing which should be avoided when planting a camellia - the risk is the sun rises and hits the buds and flowers, and any dew or frost upon them has not had time to disperse, so the sun 'burns' the flowers and buds, causing browning/yellowing.