fruit of the poisonous tree

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun US, law, idiomatic Evidence that has been obtained as the result of an illegal act on the part of law enforcement personnel (such as a warrantless search, or continued questioning of a witness who has invoked the right of counsel), and which is therefore excluded from being admitted as evidence in a trial.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a rule that once primary evidence is determined to have been illegally obtained any secondary evidence following from it may also not be used