Kate will attend tonight's show in her role as an official ambassador of Britain's Olympic and Paralympic team, St James' Palace said.
A spokesman said:
The Duchess of Cambridge will tonight attend the BBC's Sports Personality of the Year awards at the ExCeL London, where Her Royal Highness will present the Lifetime Achievement and the main Sports Personality of the Year awards.
The Duchess will attend the event in response to a long-standing invitation from the BBC.
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Marjorie and Michael Cawdery, both 83, are understood to have been stabbed to death in a killing their family called "incomprehensible".
Detectives have launched a murder investigation following the shooting in Brent, north west London on Friday night.
Hot and humid throughout the Bank Holiday Weekend.