Prince Albert II of Monaco and his wife Charlene welcomed their twin babies to the world on Wednesday, and announced that Jacques, born two minutes after his sister Gabriella, will be next in line to the throne.
Cannons roared and church bells rang in the tiny Mediterranean principality at the announcement of the births of the next head of the 700-year-old House of Grimaldi and his sister.
Gabriella Therese Marie was born at 5.04pm local time followed by Jacques Honore Rainier at 5.06pm, the palace said in a statement, adding that the newborn babies and their South African mother were "doing well".
The babies replace Prince Albert's sister Caroline as heirs to the throne, and will likely draw a line on rumours of the royal couple's supposedly rocky relationship.
The prince's late father, Rainier III, had rewritten the Constitution as he became ill in 2002 so that one of his daughters could inherit the throne if his son - who had fathered two children out of wedlock - failed to produce a legitimate heir.
But Prince Albert, 56, finally married former Olympic swimmer Charlene, 36, in 2011 - 11 years after the pair first met.
The birth of the twins was celebrated with 42 cannon shots, 21 for each child, fired from an old fort overlooking the sea.
Church bells also rang for 15 minutes, followed by boat horns, as Monegasques toasted their future absolute rulers.
The gender of the twins had been kept a secret during Princess Charlene's pregnancy, even from their father, who said that he wanted to be surprised.
Prince Albert - whose late mother was Hollywood actress Grace Kelly - had earlier said that if the twins were a boy and a girl, it would be the boy who would succeed him.
According to tradition, an official birth announcement signed by the prince will be displayed at the entrance to the palace and the public will be invited to sign a book of congratulations.
Monegasques have also been encouraged to fly the principality's red-and-white flag from their homes until the day the twins are formally presented to the nation by the royal couple from the palace balcony.
Under Monaco's inheritance laws, neither of Prince Albert's illegitimate children have any claim to royal titles or to be considered as his heirs, because they were born outside of marriage.
They do, however, have legal rights to a share of his huge personal fortune, estimated by Forbes magazine to exceed US$1 billion (S$1.3 billion).