-- President Obama is extending birthday greetings to Queen Elizabeth over lunch today, one day after Britain's longest-serving monarch turned 90.
The queen and prince greeted the president and first lady as their helicopter touched down at the castle. The first lady was dressed in a purple Oscar de la Renta dress and the queen in pale blue. The four then climbed into a car, with Prince Philip in the driver's seat, and drove together to the castle. Once inside, the four posed for photos before the press.
This is Obama's third and likely final visit with the queen as president. During the Obamas' first trip to London in 2009, the first lady caused a stir when she breached royal protocol by hugging the queen.
The president and first lady will also dine with the younger generation of royalty while in London. Later this evening, Prince William and his wife, Kate Middleton -- the Duke and Duchess of Cambridge -- and Prince Harry will host the Obamas for dinner at Kensington Palace.