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 has announced that the big moment, expected in December, will be celebrated with church bells ringing and 42 cannon shots.

• Obama extends US involvement in Afghanistan
• Woman freed after wrongful conviction for murder
• 28 dead in Kenya bus attack

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

Prince Albert told BFM-TV on Saturday 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.