Prime Minister Najib Abdul Razak has sent a letter of demand to Taiping MP Nga Kor Ming over an alleged libel Facebook posting about his wife Rosmah Mansor.

The letter was sent by the legal firm Hafarizam, Wan Aisha & Mubarak to Nga's office in Taiping this afternoon.

It is understood that the matter concerns a posting that depicted Najib's wife Rosmah Mansor as the prime minister.

The letter demanded an apology from the DAP national assistant treasurer, failing which a suit would be filed.

This is the fifth legal action taken by Najib, three of which are against Pakatan Rakyat MPs, including Nga ( left ).

Last month, the PM filed a defamation suit against Petaling Jaya Utara MP Tony Pua for a speech that the DAP national publicity secretary made on Najib's pet project , 1Malaysia Development Bhd (1MDB).

Najib, who is also BN chairperson, also sent a letter of demand to Pandan MP and PKR vice-president Rafizi Ramli, for alleging the PM and his wife manipulated the nation's petrol prices for their own benefit.

The Umno president also filed a suit against Malaysiakini last year for putting up two articles, culled from comments made by the news portal's readers, on the short-lived Terengganu menteri besar imbroglio.

The PM also sent a letter of demand to PAS and its party organ Harakah in February over an online article it published on his stepson’s foray into producing Hollywood movies.