Prince George, the son of the Duke and Duchess of Cambridge, Prince William and Kate Middleton, was granted that exception on Friday during a rare meeting at Kensington Palace in London.

Smartly dressed in a white dressing gown and slippers, two-year-old George played on his rocking horse, a gift from the palace's distinguished guests, after meeting Barack and Michelle Obama.

A photograph was released of the group chatting in a drawing room before dinner with reports the young prince was allowed to stay up 15 minutes later than usual to meet the Obamas.

His little sister, Princess Charlotte, aged 11 months, was already tucked up.

Earlier in their British trip, the Obamas met Queen Elizabeth to mark her 90th birthday at Windsor Castle.

The Queen and Prince Philip greeted the Obamas as their helicopter touched down on the manicured lawns of the castle, about 30 kilometres west of London.

After chatting briefly, they climbed into the Queen's Range Rover and the 94-year-old prince drove them to the royal residence.

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

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

Mr 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," Mr Obama said. "She is an astonishing person and a real jewel to the world and not just the United Kingdom."

Thursday's lunch was Mr 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.

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, the President recounted.

with Reuters