Prince Albert of Monaco seen with his two other children amid marriage dramas with Princess Charlene

Prince Albert II of Monaco has been photographed attending the birthday of his son born out of wedlock two decades ago.

Also at the party was Prince Albert's daughter, born to another woman.

His inclusion at the event, in Monte Carlo, comes as Prince Albert's wife Princess Charlene is believed to be living in Switzerland.

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Prince Albert was seen at the 20th birthday party of his eldest son Alexandre Grimaldi-Coste in the photo shared on Instagram by Alexandre's mother, Nicole.

He was seated next to Nicole and Alexandre.

Nicole had a five-year relationship with Prince Albert and Alexandre was born in 2003.

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Prince Albert officially recognised he was the baby's father in 2005 when news of his existence became public.

At the time the story broke, Albert was dating Charlene Wittstock. They married in 2011 and have two children, twins Prince Jacques and Princess Gabriella, eight.

Prince Albert's eldest daughter Jazmin Grace Grimaldi, 31, was also at the party.

The actress' mother is American real estate agent Tamara Jean Rotolo, who met Prince Albert II while on holidays at the Côte d'Azur, on the French Riviera, in 1991.

Prince Albert finally confirmed his daughter's paternity in 2006, when she was a teenager. Rumours had been swirling for years before the story finally broke.

Jazmin also shared a happy birthday tribute to her "amazing little brother" on social media.

Albert's two eldest children are not in line for the throne, because they were born out of wedlock.

Prince Albert's marriage to Princess Charlene has been in the headlines of late and not for the happiest of reasons.

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On Thursday, the 45-year-old deactivated her personal Instagram account, days after reports claimed she only sees her husband "by appointment" and was living in Switzerland.

It's the latest twist in the saga that is the couple's marriage, which has been plagued with rumours since they wed.

Prince Albert has consistently denied trouble, shutting down divorce rumours as recently as March as "malicious rumours peddled by the French magazine Royauté".

Earlier in the week, German and French media claimed Princess Charlene and Prince Albert rarely meet except for ceremonial or formal events.

They have been putting on a public show of a loving couple, however.

Prince Albert, 65, and Princess Charlene were last seen together on July 29, when they appeared arm-in-arm at a Red Cross ball in Monaco, but seemed uncomfortable with each other.

"They take turns looking after the children," Bild, the German tabloid newspaper said, quoting an anonymous source.

French outlet Voiciclaimed Princess Charlene failed to return to the Prince's Palace in Monte Carlo following the end of a recent family holiday on board a yacht with Prince Albert and their children.

It went on to say that Princess Charlene only sees her husband by appointment and for "ceremonial reasons".

The Prince's Palace has not commented on the reports.