Interest and penalty on undisclosed income: failure to furnish required return triggers monthly interest and possible penalty on tax determined. Failure to furnish the return of undisclosed income in search or requisition cases initiated under sections 132/132A (on or after 1 September 2024) attracts simple interest on the tax determined under section 158BC for each month or part of a month from the day after the notice period until completion of assessment. A penalty may be directed by the Assessing Officer or Commissioner (Appeals) as a proportion of the tax leviable on undisclosed income, subject to a non imposition carve out where return, tax payment, evidence and no appeal for the disclosed part are satisfied, and subject to procedural safeguards, approval requirements and limitation rules with specified exclusions and extensions.