MENON, J.

The petitioners are assessees under the Kerala General Sales Tax Act, 1963. Section 18 of the Act providing for "provisional assessment" was omitted from the statute with effect from 1st April, 1982 by the Kerala General Sales Tax (Amendment) Act, 1983 (Act 3 of 1983). The Rules framed under the statute were also amended to give effect to this change in policy, but sub-rules (7) to (14) of rule 21, authorising provisional monthly assessments, were retained with some minor modifications. And the contention of the petitioners is that these sub-rules cannot validly operate after 1st April, 1982 in view of the deletion of section 18. The sub-rules are ultra vires the statute as it now stands, it is contended.

2. Ignoring details relating to classification of dealer, rates of tax and other matters, section 5 of the K.G.S.T. Act requires every dealer to pay tax on his taxable turnover for a year. Purchase tax under section 5A is also geared to the taxable turnover for a year. "Turnover" means the aggregate amount for which goods are either bought or sold; and "taxable turnover" means the turnover on which tax is payable, after permissible deductions. Section 16(1) in Chapter V, dealing with "assessment, collection and penalty", provides that :

"The tax under this Act shall be assessed, levied and collected in such manner as may be prescribed."

Section 17(1) obliges every dealer who is liable to pay tax under the Act to submit "such return or returns relating to his turnover in such manner and within such period as may be prescribed". Section 17(3) provides for best judgment assessments, where returns and not filed or where those filed are found to be incorrect or incomplete. The relevant provisions of section 18, when the section was part of the enactment, were as follows :

"18. Provisional assessment. - (1) The tax for each year payable under any of the provisions of this Act may be assessed and levied in advance during the year and for that purpose a dealer may be required to furnish within the prescribed period either an advance estimate of his turnover for the year or such periodical returns of the actual turnover as may be prescribed.

(1A) The assessing authority may determine the amount of tax payable in respect of any period under sub-section (1) and on such assessment the dealer shall pay the sum demanded in monthly or other prescribed instalments within such time as may be fixed by such authority.

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(2) If no return is submitted by the dealer under sub-section (1) within the prescribed period, or if the return submitted by him appears to the assessing authority to be incorrect or incomplete, the assessing authority may determine the amount of tax payable by the dealer in accordance with the provisions of sub-section (3) of section 17.

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(3) If the assessing authority has reason to believe that the provisional assessment made by it for any period was based on too low a turnover or was made at too low a rate or was based on too high a turnover or was made at too high a rate, it may enhance, or reduce, as the case may be, such provisional assessment :

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(4) The assessment, levy and collection of tax under this section shall be subject to such adjustment as may be prescribed on the completion of final assessment in the manner prescribed."

Section 23(1) provides that the tax assessed shall be paid in such manner and in such instalments, if any, and within such time, as may be specified in the notice of demand. Sub-section (3) of section 23 provides for payment of penal interest :

"If the tax assessed or any other amount due under this Act or any instalment thereof is not paid by any dealer or other person within the time specified therefor ................"

Section 34, as it originally stood, provided for appeals to the Appellate Assistant Commissioner against different cla