Please go through Circular No. 42/2018 CGST dated 13.04.2018
4.1 Recovery of central excise duty, service tax or wrongly availed CENVAT credit thereof under the existing law and inadmissible transitional credit:
(a) The CENVAT credit of central excise duty or service tax wrongly carried forward as transitional credit shall be recovered as central tax liability to be paid through the utilization of amounts available in the electronic credit ledger or electronic cash ledger of the registered person, and the same shall be recorded in Part II of the Electronic Liability Register (FORM GST PMT-01).
(b) The arrears of central excise duty, service tax or wrongly availed CENVAT credit thereof under the existing law arising out of any of the situations discussed in para 3 above, shall, unless recovered under the existing law, be recovered as central tax liability to be paid through the utilization of amounts available in the electronic credit ledger or electronic cash ledger of the registered person, and the same shall be recorded in Part II of the Electronic Liability Register (FORM GST PMT-01).