As per case facts, civil appeals originated from arbitral awards concerning the interpretation and implementation of the BOCW Act and Cess Act, 1996, related to cess levy on construction costs. ... The Acts, though notified earlier, remained dormant without effective implementation machinery or Welfare Boards. Contractors did not factor this cess into their bids. NHAI later sought to recover cess, which was challenged. The appeal reached the Supreme Court as NHAI challenged High Court decisions affirming arbitral awards in favor of contractors. The question arose whether the actual operationalization of the BOCW and Cess Acts, including Welfare Board constitution and collection mechanisms, constituted 'subsequent legislation' under the contracts, entitling contractors to reimbursement. Finally, the Supreme Court upheld the High Court's and arbitral tribunals' view. It concluded that effective implementation with necessary machinery and Welfare Boards was essential, and without these, the cess could not have been levied. Arbitral awards treating such operationalization as 'subsequent legislation' were found plausible and not perverse. NHAI's appeals were dismissed, and Prakash Atlanta's appeal was allowed.