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Monaco's Prince Albert II and his wife, Princess Charlene (L) attend the Army Parade, as part of the official ceremonies for the Monaco National Day in Monte Carlo, Monaco, 19 November 2014. Princess Charlene of Monaco is due to give birth to twins at the end of the year. Image Credit: EPA

Prince Albert II and Princess Charlene of Monaco are expecting twins next month, and the first one born will be the prince’s royal heir.

The palace of the Riviera principality advised citizens on Saturday that the big moment, expected in December, will be celebrated with church bells ringing.

The palace says there will be 21 cannon shots for each baby and bells tolling for 15 minutes, followed by boat horns.

Prince Albert told BFM-TV that the first child born will be heir to the throne — unless a boy and a girl are born, in which case the boy becomes royal heir.

Albert, 56, who has two children out of wedlock, married Charlene, a South African swimmer now 36, in 2011.