Since Prince George's birth in July, the Duchess of Cambridge has rarely been seen in public as she adjusts to life as a mother.
In September, Prince William and Kate spent their first night away their son to attend a black-tie awards ceremony.
The Duchess had previously made her first surprise public appearance since George's birth when she joined her husband as he started the Anglesey Coastal Ultra Marathon in August.
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Ursula Keogh, 11, went missing on Monday around 3.30pm. She was dressed in her school uniform.
"My sincerest apologies to everyone who purchased tickets and were planning to come to the upcoming shows."
His comments at the Defence and Security Forum on Monday came after a £20 billion black hole in the budget for the next decade emerged.