Paul Chambers became an Internet free-speech cause celebre when he was convicted and fined in 2010 for using Twitter to say he would blow Robin Hood Airport in northern England "sky high" if his flight, due to leave a week later, was delayed.
He insisted he was joking, but a court convicted him of sending menacing or offensive messages.
High Court Judges on Friday overturned the conviction, saying there was no evidence to suggest that anyone who saw the tweet found it to be menacing or alarming.
Chambers expressed his relief after the verdict, saying it was ridiculous the case ever got so far.
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