Initial investigations show that among the seizures made by the ED on January 13 was a document similar to the one submitted in a sealed cover to the Supreme Court by the CBI in 2013, sources said. Mr Chidambaram was then finance minister in the earlier Congress-led UPA government at the centre.
The internal probe was launched after the ED sent a note to the CBI, informing it about the seizure of the report during a raid at the Jor Bagh residence of Mr Chidambaram, CBI sources said.
Last month, the ED had raided a number of premises in Delhi and Chennai in connection with a money laundering probe against Mr Chidambaram's son Karti Chidambaram in the Aircel-Maxis case.
Mr Chidambaram had called the ED raids against his son a "comedy of errors" and had claimed that the officers were "embarrassed" and "apologetic" as they could not find anything at his Delhi home.
The ED is investigating the circumstances under which approval was granted to the Aircel-Maxis foreign direct investment (FDI) case by the Foreign Investment Promotion Board (FIPB) in March 2006 when Mr Chidambaram was finance minister.
The probe agency has said the finance minister was then empowered to clear proposals up to Rs 600 crore. Proposals beyond that needed a clearance from the Cabinet Committee on Economic Affairs (CCEA). In the Aircel-Maxis case, approval was sought for FDI worth Rs 3,500 crore but clearance was not obtained from the cabinet panel, the ED claims.
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