Сленговый вариант.
It started off as criminal argot, but has worked its way into the general vocabulary to mean anyone who "tells" on someone for doing something "wrong". A sister can squeal on her broother, a student can squeal on another student, and so on.

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Actually, it's a legal term, otherwise a bit obsolete. However, all the rest is slang.
Should you text sustain slang, use "snitching", of course.
But remember that in Russian "donos" is a mainstream term, while "snitching" is slang. Besides, "snitching" (ratting, squealing, etc.) would be “стучать“ rather than “доносить“, and "a snitch" - "стукач" rather than “доносчик“.

back. Somebody blew the whistle.

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Note added at 7 hrs (2013-07-17 09:26:08 GMT)
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Spreading the rumours,of course -- I am sorry. The letter did fit when I tried fixing it.

If you want to translate the report, rather than the reporter or his action, this is a pretty standard way to say it in English: "report" preceded by a possessive.


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Note added at 2 days18 hrs (2013-07-19 21:13:15 GMT) Post-grading
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Reply to asker: Why do you say that? "Stool pigeon" is perfectly good English, as is its colloquial form, "stoolie." You can find it in legal or crime glossaries, I'm sure. But here's the standard definition from Webster's, including an example from Time magazine:

noun 1 : a pigeon used as a decoy to draw others within a net 2 : a person acting as a decoy or informer; especially : a spy living among or sent into a group to report often to the police on the activities of its members <served ... as a stool pigeon, according to his fellow prisoners, and brought about the execution of a U.S. captain -Time>

Here's a Google Books search, from which you can familiarize yourself with thousands of examples of the usage, including some fascinating accounts of its origin, which involves tying a pigeon to a stool - which was actually more like a pole.
https://www.google.com/search?q="stool pigeon"&btnG=Search B...