Bukit Aman has opened an investigation into former premier Najib Abdul Razak's claim that the Inland Revenue Board (IRB) had imposed a hefty tax penalty on him under political instructions.

Commercial Crime Department director Kamarudin Md Din said the police are investigating the case under Section 468 of the Penal Code, which is a law that deals with forgery of documents for the purpose of cheating.

This comes after Najib lodged a report two weeks ago, alleging that a certain IRB official had received instruction from a politician to fine him, despite investigations supposedly found no tax violation on Najib's side.