Two siblings of ethnic Indian origin claiming to be Hindus have demanded that Malaysian authorities change their religion on their birth certificates that show them as Muslims.

S Jeevanathan, 22, and his sister S Maneemegalay, 21, are practising Hindus and are worried about their future as they have been certified as Muslims after their father K Seshadri converted from Hinduism to Islam before they were born.

Children of Muslim parents in Malaysia are automatically certified as Muslims, and conversions are rarely permitted.

Jeevanathan said his father converted to Islam as an 18-year-old after his grandparents decided to embrace the religion.

Their father reverted to Hinduism in 1991, but was buried by Islamic authorities as a Muslim last month despite the protests of his children, the New Straits Times said on Friday.

Jeevanathan also claimed that the religious department officers had intimidated and threatened him and his sister before taking away their father's body on that ill fated day.

The siblings, registered as Hindus on their identity cards, are trying to change their Muslim status on their birth certificates but have been turned down so far by Malaysia's National Registration Department.

Malacca religious department enforcement head Rahimin Bani said the department claimed Seshadri's body based on the police report lodged by his children from his first marriage.

"According to the law, when one embraces Islam, there is no provision that allows him to take another religion or go back to his former religion," Bani said.

In 2005, religious and racial tensions were raised when an ethnic Indian mountaineering hero born a Hindu was buried as a Muslim despite his family's protests.