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.