KUALA LUMPUR - Malaysia's most powerful political party expelled two key figures for throwing their support behind the opposition just days ahead of a closely-fought election, the state news agency reported Saturday.

Prime Minister Najib Razak's United Malays National Organisation (Umno) sacked the pair after they backed veteran ex-premier Mahathir Mohamad's bid to take power in the May 9 polls at the head of an opposition alliance. Mahathir, 92, wants to unseat former protege Najib in the toughest poll campaign the country has seen in years. He has drawn attention to the alleged billions of dollars looted from sovereign wealth fund 1MDB, which Najib founded.Both Najib and the fund deny any wrongdoing. Official news agency Bernama reported Umno secretary-general Tengku Adnan Tengku Mansor saying he sacked Daim Zainuddin and Rafidah Aziz on Friday for their criticism of the party.

"When she (Rafidah) went on stage at an opposition rally, I immediately asked for the letter of expulsion to be sent to her, as well as to Daim," he said.

Tengku added that a third party stalwart and former information minister, Rais Yatim, was under investigation.

All three served under Mahathir during his 22 year rule as Malaysia's Prime Minister.

As the race tightens, police on Thursday said Mahathir was under investigation for allegedly spreading "fake news" after claiming his plane was sabotaged to prevent him heading to the island of Langkawi to formally register his candidacy for the election.

He made it to the island on another plane and Malaysia's civil aviation authority said there was a technical issue with the aircraft but rejected allegations it had been tampered with.

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

It punishes dissemination of any news deemed "wholly or partly fake" with up to six years in jail or a hefty fine.

Police have since said that another opposition figure Rafizi Ramli, vice-president of the People's Justice Party, was being investigated for remarks made against government officials on election nomination day.

Rafizi previously said a party candidate was not allowed to file his nomination papers.