The Princess of Wales was wearing the Diamond and Pearl Leaf Brooch which belonged to the Queen at the service ahead of the lying in state.
The piece features a trio of large pearls set in the centre of a pave-decorated leaf, and had been loaned to Kate by the monarch in the past.
Kate wore it at the Tyne Cot Commonwealth War Graves Cemetery in Ypres, Belgium, at a commemoration ceremony to mark the centenary of Passchendaele in 2017.
The Queen was seen wearing it on her 73rd birthday in 1999 during an official tour to South Korea.
The princess was also thought to be wearing pearl drop earrings which belonged to her namesake Diana, Princess of Wales for the service in Westminster Hall on Wednesday.
