Falling for your friend

I'm in this rocky boat with my best "girl friend" and finding it very difficult to focus on anything else at the moment. Mad about her - and over the past few months have tried to muster up the courage to ask her would she be interested in taking things further. Some days I wake up and I am on a high - I will have the guts to talk to her, and it is all going to work out. Other days I just beat myself up - why haven't I said anything? Even if I do she won't be interested, i'll hurt her, damage our friendship?

I haven't a clue how it will work out, I am sick at the thought of it but need to know and move on if it isn't right.

Basically we have known each other 3/4 years. Worked together and she was dating somebody at the time. The funny thing is I couldn't stand her at first then as we spent more time together really got to like her. Her relationship broke down about 2 years ago and she was devastated, it is really only in the last few months that she has gotten over it. Moved on to a new job but we are very much in contact - most mornings I arrive to an email from her seeing how I am, we speak on the phone regularly, and meet up about once a week. Sometimes on our own, sometimes with friends. We share everything with each other, but never talk about other guys/girls.

At the same time she is an outrageous flirt and i wonder whether she would have said something to me if she felt the same - or maybe she feels it is up to the fella to make the first move.

I think work mates believe that something is going on and I get the odd smart comment directed - I work with a lot of girls who she would have spoken to while working there and maybe they were told something.

All in all I don't know what is the best way to approach this - I really would hate to hurt her and ruin the friendship. That sounds cliched but I genuinely feel like that.

Any thoughts/advice appreciated.