Interest on excess excise collections payable from the month after default until payment; voluntary timely payment avoids interest. Interest is payable on amounts collected in excess of excise duty, at a rate fixed by the Central Government within statutory bounds, from the first day of the month after the month the amount ought to have been paid until payment. A proviso waives interest where the amount becomes payable due to a Board order and is voluntarily paid in full within a short prescribed period without reserving appeal rights; otherwise interest is charged on the whole amount including amounts already paid. Appellate changes to the determined amount adjust the interest payable accordingly.