Several authorities may have announced they are investigating controversy-plagued 1MDB but there can be no genuine probe as long as Najib Abdul Razak remains prime minister, former prime minister Dr Mahathir Mohamad says.

This, he said, is because the authorities fear investigating the 1Malaysia Development Bhd, which is under the Finance Ministry that also headed by Najib.

"There is a climate of fear which restrains them from investigating. For as long as the prime minister (Najib) is the prime minister, investigating things involving him is not going to be really possible.

"It is necessary that the prime minister relinquishes his position and authority if the auditor-general, the Public Accounts Committee (PAC) or even a royal commission is to reveal the truth," Mahathir said in his blog posting today.

In March, after mounting criticism, Najib instructed the auditor-general to investigate 1MDB, but the move was likewise criticised due to conflict of interest concerns.

All financial dealings require PM's approval

Najib is the chairperson of the 1MDB advisory board.

On May 26, it was also revealed that 1MDB's memorandum and articles of association showed that all financial dealings required the prime minister's approval.

The PAC also initiated an investigation and summoned 1MDB's former chief executive officer Shahrol Halmi and 1MDB president and executive director Arul Kanda.

However, the duo have asked for a month's extension before appearing before the bi-partisan committee.

Last Wednesday, Bank Negara joined the fray , confirming it had started a probe against 1MDB.

However, Mahathir remains sceptical that the investigations would be genuine and shed light into 1MDB, which has accumulated a massive debt of RM41.8 billion.