Hilchos Lashon Hara chapter 8, section 12
Lashon hara applies if one disparages a person to a Jew, and how much more to non-Jews in which case his sin is far worse. For besides sullying the honor of Yisrael and desecrating the name of heaven, if one tells a non-Jew that a Jew is a cheat and suchlike, unlike a Jewish listener, the non-Jew will believe the information immediately and publicize it to all. (We find something similar in Bava Basra 39b where it explains the difference between lashon hara and protesting, see there). This causes the Jew damage and distress. How much more if he informs on a Jews [to the authorities], his punishment is beyond bearing for he is counted as an informer who has the same status as a heretic and denier of the Torah and the resurrection. Gehinom dissipates but [their punishment] does not dissipate as is taught in Rosh Hashanah (17a). Therefore one must be very careful of this. If someone informs of Jews to non-Jews it is as if he blasphemed and lifted his hand against the Torah as the Choshen Mishpat rules (ch. 26).