As if news of the royal baby wasn't enough to steal her thunder, Princess Eugenie has suffered a further insult — Buckingham Palace has mistaken her for sister while spruiking wedding souvenirs.

In a post on its official Instagram account, Buckingham Palace grandly announced the sale of commemorative mugs marking last Friday's nuptials of Princess Eugenie and Jack Brooksbank.

The post informed followers that the limited edition mugs were available "only til Saturday" and could be purchased with the click of a button.

There was only one thing — the caption mistakenly referred Eugenie's older sister, Princess Beatrice.

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"This exclusive mug issued in commemoration of the wedding of HRH Princess Beatrice of York and Mr Jack Brooksbank is now available for purchase by following the link in the description," the caption said.

Princess Beatrice is the older sister of Eugenie and acted as maid of honour on her big day at Windsor Castle.

The post was swiftly deleted from the Palace's Instagram account and will no doubt have caused numerous red faces.

It has allegedly been a trying few days for Princess Eugenie, who was reportedly upset that Prince Harry and Meghan Markle broke their baby news to the other members of the royal family during the wedding reception.

Eugnie's mother Sarah, the Duchess of York, was said to be furious that the Duke and Duchess of Sussex had broken an unwritten rule of sharing big news at a wedding, taking attention away from the bride.

In a pointed tweet just 40 minutes after Prince Harry and Meghan's pregnancy news was announced, Fergie congratulated her daughter on her wedding.

As the news of the new royal baby swept the world, Ferguson — on what was also her 59th birthday — gushed that she was "so proud of Eugenie and Jack" after the lavish ceremony that saw 850 A-listers flock to Windsor.

The Duchess of York tweeted out the congratulatory message barely 40 minutes after Kensington Palace shared the baby news.