84 [Income escaping assessment.
85 147. If the 86 [Assessing] Officer 87 [has reason to believe] that any income chargeable to tax has escaped assessment for any assessment year, he may, subject to the provisions of sections 148 to 153, assess or reassess such income and also any other income chargeable to tax which has escaped assessment and which comes to his notice subsequently in the course of the proceedings under this section, or recompute the loss or the depreciation allowance or any other allowance, as the case may be, for the assessment year concerned (hereafter in this section and in sections 148 to 153 referred to as the relevant assessment year) :
Provided that where an assessment under sub-section (3) of sec­tion 143 or this section has been made for the relevant assess­ment year, no action shall be taken under this section after the expiry of four years from the end of the relevant assessment year, unless any income chargeable to tax has escaped assessment for such assessment year by reason of the failure on the part of the assessee to make a return under section 139 or in response to a notice issued under sub-section (1) of section 142 or section 148 or to disclose fully and truly all material facts necessary for his assessment, for that assessment year.
Explanation 1.—Production before the Assessing Officer of ac­count books or other evidence from which material evidence could with due diligence have been discovered by the Assessing Officer will not necessarily amount to disclosure within the meaning of the foregoing proviso.
Explanation 2.—For the purposes of this section, the following shall also be deemed to be cases where income chargeable to tax has escaped assessment, namely:—
            ( a)       where no return of income has been furnished by the assessee although his total income or the total income of any other person in respect of which he is assessable under this Act during the previous year exceeded the maximum amount which is not chargeable to income-tax;
            ( b)       where a return of income has been furnished by the assessee but no assessment has been made and it is noticed by the Assessing Officer that the assessee has understated the income or has claimed excessive loss, deduction, allowance or relief in the return;
            ( c)        where an assessment has been made, but—
            ( i)        income chargeable to tax has been underassessed; or
            ( ii)       such income has been assessed at too low a rate; or
            ( iii)      such income has been made the subject of excessive relief under this Act; or
            ( iv)      excessive loss or depreciation allowance or any other allowance under this Act has been computed.]