"Worse comes to worst" or "worst comes to worst"

Which is correct: worse comes to worst or worst comes to worst? The former seems more logical but the latter is what appears in Oxford Advanced Learner's Dictionary.

The expression should be:

If the worst comes to the worst, ...

It means "if the worst thing that can happen does happen...".

Contracting it without the definite articles doesn't seem to me to justify 'worse comes to worst'; it should still be 'worst comes to worst'.

According to the Cambridge Dictionaries Online, the following forms of the idiomatic expression are correct:

• British English: if the worst comes to the worst
• American English: if worse/worst comes to worst

I quote the relevant definitions of idiom and expression from my New Oxford American Dictionary to put things in perspective:

idiom: a group of words established by usage as having a meaning not deducible from those of the individual words

expression: a word or phrase, esp. an idiomatic one, used to convey an idea