Dear Sir/Madam
With deep regret, I am writing to inform you that I have given careful consideration to your offer of an internship and I have decided not to accept the position as we discussed on (date). I am indeed fortunate to have received an offer of internship with you but I have now received another offer which suits my educational interests in a better way.
Thank you for the time and consideration you have given me. I appreciate it very much and regret not being able to accept your offer.
Sincerely,