The clash between Malaysia's former and current Prime Ministers entered a new phase as Dr Mahathir Mohamad filed a suit against Prime Minister Najib Razak, alleging corruption and "misfeasance and breach of fiduciaries" in public office.

Mahathir's Mohamad's law firm revealed this today in a statement.

Najib has come under criticism over allegations of corruption linked to the debt-laden state fund 1Malaysia Development Berhad (1MDB) and deposits into his private accounts worth around US$680 million.

He has denied any wrongdoing and maintains that he did not use the funds for personal gain. He also maintains that this year he was cleared of any criminal offence or corruption.

The law firm's statement accused Najib of "carrying out various steps that were actively and deliberately taken in bad faith ... to obstruct, interfere, impede and derail the various investigations and inquiries which were being conducted by various legal enforcement agencies", Reuters reported.

Mahathir joined several opposition leaders this month to sign a "Citizen's Declaration", calling for Najib's resignation.

In the lawsuit filed in Kuala Lumpur High Court, Mahathir was joined by Khairuddin bin Abu Hassan and Anina binti Saadudin, former members of the United Malay National Organisation (UMNO) which Najib heads.

(With inputs from Reuters)