Prime Minister Najib Razak faces allegations that billions of dollars were plundered from a state-owned investment fund he founded.
The accusations, which emerged last year, include the revelation that Najib received at least USD 681 million in deposits to his personal bank accounts in 2013.
"We have agreed to this because we want to cooperate towards a common goal for a change of government for Malaysia," the country's former leader Mahathir Mohamad was quoted as saying by The Star after meeting with key opposition figures.
Mahathir, who was the premier for 22 years, quit the ruling United Malays National Organisation (UMNO) in February, saying it had degenerated into an organisation whose sole purpose was to protect Najib.
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He had, until recently, been an implacable foe of UMNO opponents throughout his leadership and after.
The 91-year-old has driven the push to oust the prime minister but is not expected to helm the new party and coalition.
But the premier tightened his grip on power and has purged ruling-party critics, curbed investigations and cracked down on reporting of the financial scandal.
Najib and the state investment fund 1Malaysia Development Berhad (1MDB) deny any wrongdoing.
Malaysia's opposition has been in disarray -- with infighting and squabbles -- since the jailing of former opposition leader Anwar Ibrahim for sodomy last year.