On Wednesday the Queen became the longest-reigning British monarch, but to mark the day she has chosen to celebrate in private with her husband Prince Philip. She will be joined by her grandson Prince William and his wife the Duchess of Cambridge, née Kate Middleton and while all attention is trained on Her Majesty, the Duchess will also have reason to celebrate.
According to reports, Kate will receive the Royal Family Order of Queen Elizabeth II – the highest honour that can be bestowed upon a female member of the royal family.
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Kate is expected to receive the Royal Family Order of Queen Elizabeth II during her visit to Scotland
Kate is expected to travel to Balmoral with William, where the Queen and Prince Philip are currently residing, to receive the honour and mark her grandmother-in-law's big day.
The Order she is tipped to receive is given to female royals directly from the Queen and serves as a personal memento rather than a state decoration, although it can be worn during state occasions.
Because there is no formal announcement regarding whether the honour has been bestowed, royal watchers will have to wait until Kate's expected attendance at next month's state dinner for China's President Xi Jinping to see whether she will be wearing it.
Sophie Wessex is pictured wearing the Order to Princess Madeleine of Sweden's wedding in 2013
Previous recipients include William's late mother Diana, Princess of Wales and Prince Charles' wife the Duchess of Cornwall. Prince Edward's wife the Countess of Wessex has also been pictured wearing the Order to the wedding of Princess Madeleine of Sweden in 2013.
The Order depicts a younger Queen Elizabeth II in evening dress, wearing the ribbon and star of the Order of the Garter. The picture is painted on ivory and bordered by diamonds, while a Tudor Crown in diamonds and red enamel sits atop. It is sewn onto a chartreuse yellow bow and is usually pinned on the left shoulder of the recipient's dress.
The Queen will mark her special day in private with Prince Philip, Prince William and Duchess Kate at Balmoral
It will be business as usual on Wednesday 9 September – the day that the Queen officially becomes the longest-reigning monarch. She and Prince Philip will carry out an official engagement to open the Scottish Borders Railway in Edinburgh, Midlothian and Tweedbank.
In the evening, the couple will be joined by William and Kate for a low-key private dinner at Balmoral. The Duke and Duchess, who have travelled to Scotland with their son Prince George and four-month-old Princess Charlotte, will be the only senior royals in attendance.
Charles will be 150 miles away at Dumfries House in Ayrshire, recording a documentary about his charity the Prince's Trust, while Camila will be in London at an event to mark the 60th anniversary of ITV.