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.
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
It should be
"If the worst comes to the worst, then..."
Which means that, if a possible worst-case scenario actually occurs
if an already bad situation progresses into a catastrophic situation.