"Incase" vs "In Case" ?

While reading something I'd written on Microsoft Word I was struck by the lack of squiggly red lines underneath the word "incase" indicating a spelling error. But after I right clicked I found the suggested synonyms are "lest" and "for fear that", which pretty much sums up the word/phrase "in case". I've always thought it was "in case", am I wrong or is MS Word right? Is incase a word? My Firefox spell check is telling me it's not.