Johor Crown Prince Tunku Ismail Sultan Ibrahim should stop commenting on matters beyond his knowledge, said Dr Mahathir Mohamad.

Though he refrained from criticising Johor ruler Sultan Ibrahim Sultan Iskandar, the prime minister, however, did not mince words when it came to the heir to the throne.

"I don't want to comment on the sultan because if I say anything that is not good, it's not nice because he's the sultan.

"But this TMJ (Tunku Ismail) is a little boy… He is stupid because he doesn't know what is happening.

"So don't talk. When you don’t know anything, don’t talk," he told a special media interview in Putrajaya this afternoon.

Mahathir was asked to comment on Tunku Ismail's statement that his father's good deeds, such as giving lands to the people and government, were not highlighted.