If there is a list somewhere of the ultimate cop-offs, this should top the list. All this statement does is get you out the relationship without an explanation. If you have dated this girl for an extended amount of time, you owe her some type of explanation. She will agonize over the situation, questioning everything she did to try to figure out what she did wrong regardless of if it was her fault or not.
Girls tend to have selective hearing and will latch on to what she wants to hear. If she has any feelings at all for you, she will only hear the “I love you” part and think that the door is still cracked for reconciliation.