2-year-old Prince George stays up late to meet Obama...

US President Barack Obama flew to Queen Elizabeth's castle on Friday to wish happy birthday to the world's oldest monarch a day after she turned 90, calling her one of his favourite people.

The queen and her husband Prince Philip greeted the President and US first lady Michelle Obama as their helicopter touched down on the manicured lawns of Windsor Castle, the oldest and largest inhabited castle in the world located about 32 km to the west of the British capital.

After chatting briefly, they climbed into the queen's Range Rover and the 94-year-old Philip drove them to the royal residence, which has been a family home for British monarchs for nearly 1,000 years, as trumpets sounded in the distance.

"I have to say I have never been driven by a Duke of Edinburgh before, but I can report it was very smooth riding," Obama later told reporters.

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"The queen's been a source of inspiration for me like so many people around the world. She is truly one of my favourite people."

The Obamas later had dinner with a younger generation of royals, the queen's grandson Prince William, his wife Kate and his brother Prince Harry, at Kensington Palace in London.

Obama said he had offered his own personal congratulations and presented the queen with a photo album chronicling her visits to the United States and previous presidential meetings, beginning in 1951 when she met President Harry Truman.

"Should we be fortunate enough to reach 90, may we be as vibrant as she is," Obama said. "She is an astonishing person and a real jewel to the world and not just the United Kingdom."

Thursday's lunch was Obama's third opportunity to meet the queen after a state visit with his wife in 2011, when they stayed at Buckingham Palace, and a private audience in 2009.

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The trip to Windsor almost proved too much for one tough White House staffer who was so overwhelmed at meeting the queen she almost fainted, Obama recounted.

Later, the Obamas arrived at Kensington Palace in rainy weather and were greeted by William, Kate and Harry who ducked under an umbrella to kiss Michelle Obama on each cheek. Dressed relatively casually, with no ties for the men, the party posed for a photo before heading inside.

Reporters caught a glimpse of William and Kate's eldest child, Prince George, whose birth in 2013 sparked a global media frenzy. The young prince was allowed to stay up later than usual to meet the Obamas, and spent about 15 minutes with them.

In a photograph released to the media of the group chatting in a drawing room before dinner, a rocking horse could be seen, as well as a fluffy Portuguese Water Dog toy given by the Obamas to Prince George. Bo, the White House dog, is of that breed.

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