Undoubtably and undoubtedly are both well-formed words with clear, distinct meanings, yet the former is often used in place of the latter, giving rise to the mistaken belief that undoubtably is always wrong. It’s not. The distinction between them can be subtle; something that is undoubted is not doubted, while something that is undoubtable is not capable of being doubted. The two meanings often overlap, which is why mixing up the two words is often not a serious error, yet the difference is occasionally important. For example, one might claim that in his or her opinion the Beatles are undoubtably the most influential rock band in history, yet we probably couldn’t say they are undoubtedly so, because even one who believes the Beatles to be the most influential band must acknowledge that there are rock critics who doubt this.
[B]ut a double-bill featuring two of the biggest names in contemporary jazz is undoubtably a main event. [Los Angeles Times]
But another part of his reasoning is undoubtably a judgment about Obama. [Washington Post]
There will undoubtably be tougher controls on the oil industry: one revelation of the crisis has been how poorly it was regulated. [Telegraph]