KUALA LUMPUR, Malaysia - A Malaysian sultan who was ousted as royal ruler of a northeastern state launched a legal challenge on Monday to overrule the "unconstitutional" elevation of his son as the new sultan.

In a shock announcement last week, 40-year-old regent Tengku Muhammad Faris Petra - who had been locked in a power struggle with his younger brother - was appointed as the new royal ruler.

Their 60-year-old father Tuanku Ismail Petra was ousted via an amendment to the state constitution which states the sultan shall cease to be the ruler if he is incapacitated for a period of more than one year.

But his counsel Param Cumaraswamy told reporters that despite suffering a stroke in May 2009, the former sultan was "fully aware of the environment around him" and had instructed lawyers to file the affidavit in court Monday.

"The petition is to seek the Federal Court's intervention to declare the proclaimation of Tengku Muhammad Faris Petra as sultan as unconstitutional and that he is the righful sultan," he said.

Malaysia has nine sultans who are the royal rulers and guardians of Islam in their respective states. Under a unique rotating monarchy, they each take turns to rule for five years as the nation's king.

The royals assume a ceremonial role and rarely speak publicly, so the events in Kelantan - including earlier allegations that the ailing sultan had been held in hospital against his will - caused a scandal.

The former sultan's eldest sister Tengku Merjan Sultan Yahya Petra has criticised the appointment of her nephew, saying it was unconstitutional to proclaim a new ruler while her brother was still alive.

Legal experts quoted by the Malaysian press have disagreed and said the move was legal.

Tengku Muhammad Faris and the sultan's third son Tengku Muhammad Fakhry have been publicly at odds for the past year since the younger brother was removed from the state's succession council, which determines who is the next ruler.

The Kelantan royals were embroiled in another controversy last year when the teenage wife of Tengku Muhammad Fakhry slipped away and returned to her family in Indonesia alleging abuse, rape and torture by the prince.