According to a media report, ethnic Indian Hindu woman identified as S Deepa, 29, from Jelebu, discovered that her husband, who left her 16 months ago, had converted their two children without her knowledge.
The husband alleged that Deepa had been having an affair with another man and could not take care of their children.
Citing the Federal Constitution, Bar Council president Christopher Leong said one parent did not have the right to convert the children without the other's knowledge.
Leong said the Constitution ensured that no one was required to take part in any ceremony or worship any other religion than their own.
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He said the Constitution determined that a minor's (under 18 years) religion was to be decided by both parents, in cases where they were both alive.
"Accordingly, unilateral religious conversions of any minor children in breach of this are unconstitutional," he said.
Muslim majority Malaysia has a mixed population including ethnic Indians and ethnic Chinese.