Around half an hour after the visit from the Queen, William, Kate and their baby were seen being driven away from Kensington Palace.
The Duchess was sitting in the back of the vehicle next to the baby seat while the Duke was a front-seat passenger.
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Police have stepped up patrols after the suspected targeted attacks.
The royal couple will carry out their first official engagements as husband and wife on Tuesday.
Flights from UK airports were an average of 15 minutes late in 2017.