Leonardo DiCaprio must convert to Judaism if he wants to marry Bar Rafaeli.
The Israeli-born supermodel's father, Rafael Rafaeli, has reportedly forbidden Bar from marrying outside her faith.
A source said to the New York Daily News newspaper: "He told me that if Leonardo doesn't convert to Judaism, there will be no marriage."
The 'Revolutionary Road' actor, who has been dating 23-year-old Bar since 2005, recently revealed he wants to wed her.
Leonardo, 34, said: "I absolutely believe in marriage. In saying that I realise I am contradicting everything I've said before. Yes, I want to get married and have children.
"It sounds like the ignorance of youth to me. When did I say those things? Three or four years ago? Hey, we grow up real fast."
But if he does pop the question, Leonardo won't tell anyone about it - he is notoriously secretive about his private life, something he claims is essential to his career.
He said: "Defining yourself to the public on a consistent basis is death to a performer. The more you define who you are personally the less you are able to submerge into the characters you do, and people will think, 'I don't buy him in that role.' "