first person pronouns - How common/formal is あたくし?

A while ago, I discovered that あたし has a more-formal variant: あたくし

This is obviously very similar to the relationship between わたし and わたくし. However, there are a couple of details that I'm not too sure about:

1. How common is it?

あたくし doesn't seem to be used as much as other personal pronouns; I could only find two examples of it being used on JLU SE, neither of them related to this question. It's also mentioned on this page and on Japanese Wiktionary, though it's currently absent from the English version.

2. How formal is it?

My understanding is that あたし is less formal than わたし, which in turn is less formal than わたくし. Given that あたくし is more formal than あたし, and (I assume) less formal than わたくし, how does it compare with わたし?

^{Note: It seems that わたし, わたくし, あたし, and あたくし are all represented by 私 in kanji form, so this question uses hiragana to avoid confusion.}

Answer

Wikipedia says:

(あたくしは)あたしのきどった言い方である。一般的には昭和時代の漫画やアニメなどで使用された例があるが、平常的には聞くことはまずない言い方である。特に落語家が使用する。

In my opinion あたくし (not わたくし) is typically used by Kantō, classy, pompous, elder, female celebrities, mainly in fiction. Or by someone who impersonates such a person.

I confirmed that 黒柳徹子 uses あたくし in her TV show 徹子の部屋 (video). Someone says that she is the only person who regularly uses あたくし on TV (I don't know if it's true).

Other than this, I might have heard this actually used once or twice in real life (in a party or something like that), but my memory is vague.

Regarding the formality, it cannot be used in business/formal writings, where we have to almost always stick to 私 anyway. But I also feel that 黒柳徹子 may well give a formal speech using あたくし. I guess most people won't regard あたくし as impolite, because it sounds similarly enough to わたくし. It will sound just funny, if used by a wrong person, for example, me.