Malaysian opposition leader Mahathir Mohamad is under investigation for allegedly spreading "fake news" after claiming his plane was sabotaged, police said Thursday, just days before a closely-fought election.

Veteran ex-premier Mr Mahathir, 92, is seeking to oust his former protege, Prime Minister Najib Razak, in the general election next Wednesday at the head of an opposition alliance.

As campaigning got underway at the weekend, Mr Mahathir claimed that there had been an attempt to sabotage a plane he had chartered to prevent him heading to the island of Langkawi to formally register his candidacy for the election.

He still made it to the island by taking another plane. Malaysia's civil aviation authority and the charter company said there was a technical issue with the plane but rejected the allegations of sabotage.

Kuala Lumpur police said a report had been lodged accusing Mr Mahathir of spreading "fake news" under a controversial new law, which critics say is aimed at cracking down on dissent before the poll.