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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Labour lost half its MEPs as voters drifted to parties with firm Brexit positions.
Results of the European Parliament elections are on the fronts.
Turnout was at a two-decade high for ballots across all 28 European Union countries.