"Klutz" is a fairly commonly-used word. It comes from Yiddish and means "a clumsy person". Of course, in this day and age of political correctness, you might want to avoid a word which comes from a culture not your own in case someone thinks you are making fun of it.
I can't think of a standard BrE word which suggests a clumsy or incompetent person. I certainly wouldn't use gawk or muff. "Oaf" is close.
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