стукач

a sneak {BrE}/ a snitch {AmE} /a dobber{AuE} (a person who tells people in authority when someone else does something bad) {slang disapproving} — стукач, доносчик, осведомитель, информатор; ябеда

Example: You told Mrs Cooper that it was me who tipped the paint over, didn't you - you nasty little sneak! (CALD)

an informant /an informer (someone who secretly gives information about someone to the police or someone in authority)

a dog (someone who gives information about people to the police or to another authority)

a fink{US infml old-fashioned} (a person who you dislike, especially because they give information to the police, a teacher, or a parent when someone else breaks a rule or a law)

a grass / a supergrass {UK} (a criminal who gives information about a lot of other criminals to the police, in order to get a less severe punishment)

a stoolpigeon {US infml} (someone, esp. a criminal, who helps the police to catch another criminal, usually by giving them information, informer]

a snout (someone who gives information about criminals to the police)

a nark {esp. UK} (someone who is friendly with criminals and who secretly informs the police about their activities, informer)

Стучать can also be used to indicate that someone is a snitch (стукач — literally “knocker”). The acting of denouncing someone is настучать:
Ты давай кончай с этой враждебной пропагандой, а то я на тебя в органы настучу (Come on — shut up with the enemy propaganda, or I’ll rat on you to the authorities). (Michele A. Berdy)

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a tattle-tale (US) /a telltale {UK} /a sneak/ a snitch — ябеда

Related vocabulary:
[донос]
whislte blower in
[blow the whistle]