KUALA LUMPUR - Former Malaysian Prime Minister Tun Dr Mahathir Mohamad on Monday launched a new attack in his ongoing spat against Prime Minister Najib Razak, this time mentioning Datuk Seri Najib's wife, Datin Seri Rosmah Mansor and her "lavish lifestyle".

Putting pressure again on Mr Najib to address his questions on the troubles of the 1Malaysia Development Berhad (1MDB) state-owned investment agency, Dr Mahathir reiterated again that he felt Mr Najib was no longer fit to be Prime Minister over his administration's handling of the agency.

"There are many things about his personal behaviour that I thought were not right. But I was prepared to overlook them, including he and his wife's lavish lifestyle," Dr Mahathir wrote in his latest blog post, the Malaysian Insider reported.

"But when he cannot explain where billions of ringgit have gone to and his involvement with questionable people in the management of 1MDB, I feel that he is not fit to be the prime minister of this country.

"I will explain about how he lost billions of ringgit in my next blog," he added.

In recent weeks, Dr Mahathir, has publicly berated Mr Najib on a number of issues - most of which are centred on the poor performance of the agency, which Mr Najib launched.

Dr Mahathir, Malaysia's longest-serving prime minister from 1981 to 2003, still commands much respect in the country despite having stepped down.

He has also called on the ruling Umno party to remove Mr Najib or risk losing the next general election.

The intense barrage of criticisms fired at Mr Najib by Dr Mahathir has drawn even his foes to his side. On Saturday, Democratic Action Party adviser Lim Kit Siang said he was ready to put aside past problems and work with Dr Mahathir, if that is what it took to save the country.

But Dr Mahathir had on Sunday ruled out working with the opposition to "bring Najib down", saying the problem is within Umno itself.