Former Malaysian leader Mahathir Mohamad has filed a suit against Prime Minister Najib Razak, alleging corruption and "misfeasance and breach of fiduciaries" in public office, his law firm said in a statement.
Mr 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 $US680 million.
He has denied any wrongdoing, maintains that he did not use the funds for personal gain and this year he was cleared of any criminal offence or corruption.
The law firm's statement accused Mr 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".
Mr Mahathir joined several opposition leaders this month to sign a "Citizen's Declaration", calling for Mr Najib's resignation.
In the lawsuit filed in Kuala Lumpur High Court, Mr Mahathir was joined by Khairuddin bin Abu Hassan and Anina binti Saadudin, former members of the United Malay National Organisation (UMNO) which Najib leads, Malaysian media reports.