Hilchos Lashon Hara chapter 8, section 12

Know also: lashon hara is forbidden even when he speaks against a person in front of Jews. And all the more so if he speaks against him in front of non-Jews. Then his sin is much bigger. Besides shaming the honor of the Jewish people and desecrating the honor of Heaven, he also causes great harm to the other person. If he tells a Jew something bad about someone, the Jew will not believe him right away. But if he tells a non-Jew about a Jew, the non-Jew will believe him right away and spread it to everyone, and this will cause the person harm and distress.

And all the more so if he goes and informs on a Jew to non-Jews: his sin is too heavy to carry. He is counted among the informers (malshinim), and his status is like that of the apikorsim and those who deny the Torah and the resurrection of the dead. For them, Gehinom comes to an end, but they do not. So one must be very careful about this. Someone who goes and informs on Jews to them is as if he insulted and cursed the Torah of Moshe Rabbeinu, peace be upon him.