Mukesh Singh v. State (Narcotic Branch of Delhi) [(2020) 10 SCC 120] (Supreme Court, 5-judge bench)
In a reference from a three-judge bench, a Constitution Bench considered the effect on trial of investigation being conducted by the same police officer who is the complainant. The Constitution Bench held that (para 60):
Earlier Supreme Court decisions, which had observed that the trial stood vitiated and the accused was entitled to acquittal on the grounds that the complainant and the investigating officer was the same, are to be treated as confined to their own facts and not as laying down any general proposition
The question of bias or prejudice in investigation depends on the facts of the case. The mere fact of the informant and investigating officer being the same does not vitiate the investigation as biased.
Fertico Marketing and Investment Pvt. Ltd. & Ors. v. CBI & Anr. [(2021) 2 SCC 525] (Supreme Court, 2-judge bench)
The Supreme Court held that the trial court order taking cognizance cannot be set aside merely on account of any illegality in investigation, unless the illegality is shown to have caused prejudice or brought about a miscarriage of justice. The Appellants had sought quashing of proceedings against them inter alia on the grounds that the investigating agency had erred in initiating investigation against them without obtaining prior consent under Section 6 of the Delhi Special Police Establishment Act, 1946. The Supreme Court held however that there were no pleadings by these public servants with regard to the prejudice caused to them on account of this non-obtaining of prior consent; and the investigation was thus, not vitiated.