The Duchess of Cambridge paid tribute to her husband's late mother in the stunning headpiece for the royal reception hosting 1,000 guests.
It is one of the 36-year-old's favourite headpieces, having worn the Cambridge Lover's Knot Tiara several times before.
The headpiece was made in 1914 and given to William's late mother Diana by the Queen as a wedding gift in 1981.
A beaming Kate – who had hours earlier hosted Christmas celebrations for the families of military personnel – matched the tiara with a shimmering Jenny Packham gown for the annual event.
It comes after it was revealed Meghan Markle had been involved in a row over which tiara she would wear for her wedding to Prince Harry in May.
The former US actress had initially wanted a tiara that featured emeralds but was told it would not be possible to wear on her big day as the origins of the headpiece were unknown.
And while Prince Harry is said to have told staff "what Meghan wants, Meghan gets", the Queen is believed to have warned off her grandson from using the tiara.
Meghan continued to make headlines after it was rumoured she had been involved in a "ghastly row" with Kate Middleton.
A furious Duchess of Cambridge recently slapped down Meghan after she spoke rudely to a member of Kate's staff telling her doing so was 'unacceptable'.
Recent reports have also suggested Harry and his pregnant wife are abandoning their central London base at Kensington Palace for Frogmore Cottage in the grounds of Windsor Castle.
But the troubles seemed to be far away for the royal family last night as they celebrated the annual white tie Diplomatic Corps reception.
The Queen particularly seemed in high spirits for the event, which was attended by foreign ambassadors based in the UK.
Joined by the Prince of Wales and the Duchess of Cornwall for the night of nights, the 92-year-old monarch dazzled in an Angela Kelly dress.
She wore the Queen Mary Girls of Great Britain and Ireland tiara – one of her favourites and one she wears often.
She also wore white gloves and carried a gold handbag.
The Duchess of Cornwall also dressed to the nines for the reception, wearing a a Boucheron tiara that once belonged to the Queen Mother.
Both she and the Duchess of Cambridge wore the Queen's Royal Order.
The adornment features a jewel encased image of the Queen pinned on a chartreuse yellow ribbon and worn on the left shoulder.
It's only the second time Kate has been snapped wearing the honour – previously wearing it two months ago at a state banquet honouring the Dutch royals.
But she made sure to pay tribute to Princess Diana with her tiara choise.
Queen Mary left the tiara to the Queen, who wore it at a number of events.
She then gave it to Diana as a wedding present, who wore it at a number of times – including on an official visit to Washington with Prince Charles.
Meanwhile, a pregnant Meghan Markle last night attended a service at St Luke's Church in Chelsea, giving a speech to remember Prince Harry's close friend who was killed in a car crash at just 18.
Tragic Henry was close friends with both the royal princes at the time of his death.
Harry and William are still close friends with the surviving van Straubenzee brothers and they attended Thomas van Straubenzee's wedding in 2013.
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