If ITC has been claimed under the wrong head (CGST/SGST instead of IGST), the GST law allows the correction only through the subsequent GSTR-3B, not through the portal’s amendment utility.
Since the taxpayer has opted for quarterly filing under QRMP, the rectification can be done in the next available GSTR-3B within the same financial year.
How the correction is made:
Reverse the wrongly claimed ITC (CGST + SGST) in the next GSTR-3B under “ITC Reversal – Others”.
Re-avail the correct ITC under the IGST head in the same return, provided the invoice is eligible and the time limit of Section 16(4) has not lapsed.
Ensure your working papers clearly record the error and the correction, in case of later verification.
Since the law does not allow cross-utilization or reclassification directly on the portal, this is the only practical and accepted method of rectification.
If the taxpayer wants to avoid compliance mistakes going forward, it’s advisable to have the ITC classification reviewed periodically by a professional familiar with GST reconciliations and return cycles.