The admissibility of evidence in Courts in India is dependent on its relevancy as per the provisions of Indian Evidence Act.
Illegality or impropriety in obtaining the evidence will not affect its admissibility, if it is otherwise relevant.
Test of admissibility of evidence lies in its relevancy and not on how it was obtained.
In Kuruma v The Queen  AC 197 the Privy Council laid down that if the evidence is admissible, the court is not concerned how it was obtained. The privy council observed :
"...the test to be applied in considering whether evidence is admissible is whether it is relevant to the matters in issue. If it is, it is admissible and the Court is not concerned with how the evidence was obtained".
In this case, evidence obtained by illegal search of body of a person was not eschewed by the court.
These observations of the privy council were quoted with approval by the Supreme Court in Pooran Mal v Director of Inspection AIR 1974 SC 348. After quoting the above observations of the Privy Council, the SC observed as follows:
"It would be thus seen that in India, as in English, where the test of admissibility of evidence lies in its relevancy, unless there is an express or necessarily implied prohibition in the Constitution or other law, evidence obtained as a result of illegal search or seizure is not liable to be shut out".
That was a case dealing with a challenge that a search and seizure were in contravention of the provisions of Section 132 of the Income Tax Act. Rejecting the challenge, the Supreme Court held that even assuming that the search and seizure were illegal, still the material seized was liable to be used in evidence.
In an earlier decision in Magraj Patodia v R K Birla and others AIR 1971 SC 1295, the Supreme Court held that the fact that a document was procured by illegal means will not bar its admissibility, if it is relevant otherwise. It was held that provisions in Code of Criminal Procedure relating to search and seizure are to be treated as guidelines and even if there is any violation, the Court can accept the evidence.
The legal position regarding the question of admissibility of the tape recorded conversation illegally collected or obtained was discussed in R.M. Malkani v. State of Maharashtra (1973) 1) SCC 471. In that case, the Court clarified that a contemporaneous tape record of a relevant conversation is a relevant fact and is admissible as res gestae under S.7 of the Evidence Act, even if it is recorded illegally. The Court observed in Malkani :
"There is warrant for the proposition that even if evidence is illegally obtained it is admissible. Over a century ago it was said in an English case where a constable searched the appellant illegally and found a quantity of offending article in his pocket that it would be a dangerous obstacle to the administration of justice if it were held, because evidence was obtained by illegal means, it could not be used against a party charged with an offence. See Jones v. Owens (1870 (34) JP 759). The Judicial Committee in Kuruma v. R. (1955 (1) All ER 236) dealt with the conviction of an accused of being in unlawful possession of ammunition which had been discovered in consequence of a search of his person by a police officer below the rank of those who were permitted to make such searches. The Judicial Committee held that the evidence was rightly admitted. The reason given was that if evidence was admissible it matters not how it was obtained. There is of course always a word of caution. It is that the judge has a discretion to disallow evidence in a criminal case if the strict rules of admissibility would operate unfairly against the accused. That caution is the golden rule in criminal jurisprudence."
The SC has repelled the contention that obtaining evidence illegally by using tape recordings or photographs offend Articles 20(3) and 21 of the Constitution of India (Yusufalli Esmail Nagree v. The State of Maharashtra, AIR 1968 SC 147).
But the suggestion of Law Commission has not been accepted.
Exception in cases of illegally recovered contraband
This rule cannot be applied to use illegally recovered contraband as evidence of possession. Statutes like Narcotic Drugs and Psychotropic Substances Act, Abkari Act etc, raise a presumption of guilt if possession of illegal article is established. Such laws have stringent provisions for search and seizure, as the mere possession of the article can lead to punishment. "Unlawful possession" of the contraband is necessary for recording conviction under the NDPS Act.
In such cases, deviations from procedure of search and seizure will materially affect the prosecution case in relation to proving possession.This has been clarified by the Constitution Bench in State of Punjab v Baldev Singh (1999) 6 SCC 172 by explaining the dictum in Pooran Mal.
"The judgment in Pooran Mals case (supra), therefore, cannot be understood to have laid down that an illicit article seized during the search of person, on prior information, conducted in violation of the provisions of Section 50 of the Act can be used as evidence of unlawful possession of the illicit article on the person from whom that contraband had been seized during an illegal search", said the Court in Baldev Singh.
The issue reached Constitution Bench in view of conflicting decisions regarding the impact of breach of Section 50 of NDPS Act. Few decision held that even in cases of breach of Section 50 NDPS Act, evidence can be admitting, by applying the dictum in Pooran Mal.
The Constitution Bench held that the dictum in Pooran Mal cannot be applied in cases of unlawful seizure of contraband.
"the illicit drug or psychotropic substance seized in an illegal search cannot by itself be used as proof of unlawful conscious possession of the contraband by the accused. An illegal search cannot also entitle the prosecution to raise a presumption under Section 54 of the Act because presumption, is an inference of fact drawn from the facts which are known as proved. A presumption under Section 54 of the Act can only be raised after the prosecution has established that the accused was found to be in possession of the contraband in a search conducted in accordance with the mandate of Section 50".
The Court held that hat an illicit article seized from the person of an accused, during search conducted in violation of the safeguards provided in Section 50 of the Act, cannot by itself be used as admissible evidence of proof of unlawful possession of the contraband on the accused.
It was clarified that any other material/article recovered during that search may, however, be relied upon by the prosecution in other/independent proceedings against an accused notwithstanding the recovery of that material during an illegal search and its admissibility would depend upon the relevancy of that material and the facts and circumstances of that case.
This dictum is based on the principle in criminal jurisprudence that when a safeguard or right is provided favouring the accused, it has to be strictly followed(State of Delhi v Ram Avtar @ Rama AIR 2011 SC 2699)
Justice Benjamin Cardozo, stated that under the rule, "The criminal is to go free because the constable has blundered."
A major criticism of the Fourth Amendment exclusionary rule is that the text of the Fourth Amendment does not indicate that illegally seized evidence must be excluded. The argument is that the person obtaining illegal evidence must be made to face legal consequences; but the evidence should not be excluded, as the concern of the court is to be factually correct.
This rule of excluding 'fruits of a poisonous tree' has a limited application in India under Sections 24, 25 and 26 of the Indian Evidence Act which bar the admissibility of confessions made to police officers, custodial confessions etc.
(Image sourced from here)
Read the report of 94th Law Commission on illegally obtained evidence