Regardless of the reason, sending a regret response to a wedding invitation can be a delicate task. It is important to make sure the bride and groom understand that you consider their wedding to be a momentous occasion and special day. Let them know that you're truly sorry that you won't be there as they start the next chapter in their lives together. A thoughtful note can help soften the blow.
Send a formal response if the invitation is formal. According to mygoodgreetings.com, your note should be written in blue or black ink on cream coloured paper, and should be kept short, simple and to the point. If the wedding invitation is not formal, your response can be written more casually on a small card.
Make sure your note is sincere. It should say something like, "Dear (bride and groom), it is with sincere regret that I will be unable to attend your wedding. Unfortunately, I will be (reason for missing the wedding). I wish you the best on your special day and in the future. With Regret, (your name)."
If you'd like the note to be more formal, or if it's for a boss or someone you aren't as close to, you can write: (Your Name) regrets that he/she will not be able to attend the wedding of Mr. and Mrs. (groom's last name) on (wedding date).
Send your response promptly. It's proper etiquette to respond quickly when sending in an RSVP to a wedding, but even more important when declining the invitation. Try to send back the RSVP within a couple of days.
Follow up your RSVP with a wedding gift. Choose a gift from the bride and groom's registry or come up with your own idea.