President and first lady d-rop in on Queen Elizabeth II to wish her a happy birthday

US President Barack Obama plunged into a whirlwind of royal socialising on Friday that began over a birthday lunch with Queen Elizabeth II and ended at a dinner hosted by the trio of young royals who represent the future of the British monarchy.

President Barack Obama, Prince William, Duke of Cambridge and First Lady Michelle Obama talks with Prince George at Kensington Palace in London, England. Picture: Pete Souza/The White House via Getty Images.Source:Getty Images

Obama, accompanied by his wife, Michelle, arrived by helicopter on the verdant grounds of Windsor Castle, the sprawling, centuries-old royal residence and tourist lure located just west of London whe-re the Queen celebrated her 90th birthday a day earlier.

With a patterned scarf tied around her head in a light drizzle, the Queen climbed f-rom the dark blue Range Rover that her husband, Prince Philip, drove to the landing area for Britain’s oldest and longest-serving monarch to welcome her third US president to the castle.

Britain's Prince George of Cambridge playing on a rocking horse given to him by President Obama.Source:AFP

The couples shook hands before climbing into the vehicle — ladies in the back seat — for the short ride to the castle.

Inside, the queen led the group into a sitting room warmed by a fire and asked the president whe-re he wanted to sit.

“The Queen’s been a source of inspiration for me,” Obama said later at a news conference alongside British Prime Minister David Cameron.

“She is truly one of my favourite people ... an astonishing person.”

US President Barack Obama talks with Prince William, Duke of Cambridge as Catherine, Duchess of Cambridge plays with Prince George as First Lady Michelle Obama talks with Prince Harry at Kensington Palace in London, England. Picture: Pete Souza/The White House via Getty Images.Source:Getty Images

Obama capped the day with a trip to Kensington Palace, whe-re he and the first lady were invited for dinner with Prince William, his wife, Kate, and his brother, Prince Harry.

Obama, wearing an open-collar shirt with his suit, held an umbrella for his wife, dressed in a camel-coloured ensemble.

They were greeted by William, Kate and Harry, and posed for photographs before stepping inside the royal residence.

Prince William, Duke of Cambridge speaks with US President Barack Obama as Catherine, Duchess of Cambridge speaks with First Lady of the United States Michelle Obama and Prince Harry in the Drawing Room of Apartment 1A Kensington Palace.Source:Getty Images

William and Kate’s nearly three-year-old son, George, was spotted just inside the door.

Later pictures released by the White House showed George shaking hands with Obama wearing a white gown and slippers.

Earlier at the Windsor Castle luncheon, the queen donned a periwinkle blue while Mrs Obama wore an Oscar de la Renta print dress topped with a dark purple Narciso Rodriguez coat.

Catherine, Duchess of Cambridge speaks with First Lady of the United States Michelle Obama.Source:Getty Images

The Obamas brought as a gift a photo diary of the Queen’s many visits with US presidents and first ladies.

The album opens with her 1951 visit when, as Princess Elizabeth, Duchess of Edinburgh, she toured George Washington’s estate at Mount Vernon in Virginia and met President Harry Truman at the White House. She has not visited Obama at the White House.

Prince William, Duke of Cambridge speaks with US President Barack Obama in the Drawing Room of Apartment 1A Kensington Palace.Source:Getty Images

Obama broke up the royal holiday by stopping at 10 Downing Street for talks with Prime Minister David Cameron about the Islamic State group, Afghanistan, Syria, Libya, the global economy, Russia’s stance toward Ukraine, politics and other issues.

They also discussed Britain’s possible exit f-rom the European Uni-on, or Brexit, which Obama publicly argued against.

Queen Elizabeth warmly greets President Barack Obama and First Lady Michelle Obama after they land at Windsor Castle to celebrate the occasion of her 90th birthday.Source:Getty Images

Cameron is leading the campaign to keep Britain in the 28-nation EU.

At the news conference, Obama said solving world challenges “requires collective action” as he argued for why Britons should vote to stay in the alliance. He said membership amplifies British influence around the world.

Cameron is facing opposition f-rom within his Conservative government and widespread scepticism among voters about the benefit of EU membership. 

The President and the First Lady are currently on a brief visit to the UK whe-re they had lunch with Queen Elizabeth II at Windsor Castle.Source:Getty Images

Backers of those who support Britain’s exit have accused Obama of hypocrisy and meddling.

Obama said the eventual outcome a pending June 23 referendum would not change the “special relationship” that exists between the US and the UK.

He cited the case of a top aide, deputy chief of staff Anita Decker Breckenridge, who has worked for him since before he was first elected.

Obama said his aide’s one request was to accompany him to Windsor, if he ever went, on the slight chance that she could get to see the queen.

Prince Harry, US President Barack Obama, First Lady Michelle Obama, Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge pose as they attend a dinner at Kensington Palace.Source:Getty Images

Obama said the Queen graciously included Breckenridge in the group of people she greeted as they emerged f-rom lunch.

And, he said that Breckenridge, “who is as tough as they come, almost fainted.”

“That’s the special relationship,” he said.

“We are so bound together that nothing’s going to impact the emotional and cultural affinities between our two countries. So I don’t come here in any way suggesting that that is impacted by a decision the people of the United Kingdom might make.”

After lunch with the Queen, President Barack Obama flew to 10 Downing Street to meet with British Prime Minister David Cameron.Source:Getty Images