"Where a court lacks inherent jurisdiction in passing a decree or making an order, a decree or order passed by such court would be without jurisdiction, non est and void ab initio. A defect of jurisdiction of the court goes to the root of the matter and strikes at the very authority of the court to pass a decree or make an order. Such defect has, always been treated as basic and fundamental and a decree or order passed by a court or an authority having no jurisdiction is a nullity. Validity of such decree or order can be challenged at any stage, even in execution or collateral proceeding."
Balvant N. Viswamitra & Others Vs.Yadav Sadashiv Mule (dead) Through LRs. & Others (2004) 8 SCC 706
"It is settled law that a decree passed by a court without jurisdiction on the subject - matter or on the grounds on which the decree made which goes to the root of its jurisdiction or lacks inherent jurisdiction is a coram non judice. A decree passed by such a court is a nullity and is non est. Its invalidity can be set up whenever it is sought to be enforced or is acted upon as a foundation for a right even at the stage of execution or in collateral proceedings. The defect of jurisdiction strikes at the very authority of the court to pass decree which cannot be cured by consent or waiver of the party....."
Chrinjilal Shrilal Goenka (deceased) Through LRs. Vs. Jasjit Singh & Others (1993) 2 SCC 507