beg your pardon, but

(I) beg your pardon, but...

 and Begging your pardon, but...
Please excuse me, but. (A very polite and formal way of interrupting, bringing something to someone's attention, or asking a question of a stranger.) Rachel: Beg your pardon, but I think your right front tire is a little low. Henry: Well, I guess it is. Thank you. John: Begging your pardon, ma'am, but weren't we on the same cruise ship in Alaska last July? Rachel: Couldn't have been me.
See also: beg, but
References in classic literature ?
I beg your pardon, but professional profundity must be exact.