Johor DAP has apologised to Johor ruler Sultan Ibrahim Sultan Iskandar after a party member made a "rude" comment against Crown Prince Tunku Ismail Sultan Ibrahim.

State DAP secretary Tan Hong Pin said the Facebook posting by its Pontian councillor Kamarul Ariffin Harris Fadzillah was rude and did not reflect a citizen who respects the royal institution.

"Johor DAP apologises to his majesty the Johor sultan and the people for the irresponsible behaviour of our member who was rude towards the institution of the constitutional monarchy," said Tan in a statement today.

Tan, who is also the Johor exco in charge of local government, science and technology, said Kamarul should have expressed his views in a more diplomatic manner.

In the posting, Kamarul had questioned Tunku's Ismail's recent apology for any mistakes he had made last year, and asked if it was to distract from the conversion of Pulau Kukup to sultanate land.

Kamarul later deleted the post and apologised, but said his stance on the issue remains the same.

Tan said: "As a party member and local councillor, he should have been more aware of his language, manners.

"While DAP firmly upholds the Federal Constitution and champions the freedom of speech, the principle and struggle should not be an excuse to make irresponsible comments.

"Any comments should be made in accordance with strong diplomatic etiquette and integrity."

He added that the party constantly monitors the discipline of its members.

"DAP reiterates its stance that it upholds the Federal Constitution and constitutional monarchy on top of preserving the harmony and well-being of the people."