Kate and George were spotted driving into Buckingham Palace, where they spent part of the morning. It is thought that following the short visit, they headed back to Kensington Palace, where a party will be held later in the evening.
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The Duke and Duchess of Cambridge will hold the afternoon tea party inside their apartment however Her Majesty's husband, the Duke of Edinburgh, is thought not to be attending due to other engagements.
Close family members, friends, and the Prince's godparents will all celebrate George's birthday with Kate and William, and the Queen is also expected to attend.
George's grandparents Prince Charles and the Duchess of Cornwall are also thought to be skipping the party as they are in Scotland on annual leave.
Royal fans rose early to visit the palace gates on Tuesday. One fan, John Loughrey, was one of the well-wishers heading to the official residence of Prince William and Kate to decorate the famous gates with birthday balloons, banners and a Union Jack flag.
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Details of the palace bash came after speculation Kate's mother Carole Middleton hosted a Peter Rabbit-themed party for the family on Friday at their home outside Bucklebury in Berkshire.It's been reported his gifts at the youngster's party will include a wooden train set from his parents and a cake from his Spanish nanny Maria Borrallo.