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By Alexander Smith

LONDON — President Barack Obama arrived for lunch with Britain's 90-year-old Queen Elizabeth II on Friday, ahead of scheduled talks with the U.K. prime minister on what he sees as a defining moment in the U.K.'s history.

Barack and Michelle Obama landed in the grounds of Windsor Castle in Marine One and were greeted by the Queen and her husband, Prince Philip, the Duke of Edinburgh.

The four chatted briefly on the grass, before the 94-year-old prince drove them a short distance to the castle, the president sitting in the front passenger seat, the Queen and first lady sat in the back.

The Queen celebrated her 90th birthday Thursday and is the world's oldest reigning monarch.

After lunch Obama was set to meet British Prime Minister David Cameron for bilateral talks. Among the topics will be Britain's upcoming referendum over whether it should remain in the European Union.

Obama has already weighed into the June 23 vote, writing a spirited article in Britain's Daily Telegraph newspaper that argued Britain would be much better off in.

Obama walks down the steps of Air Force One at London's Stansted Airport on Thursday.Kirsty Wigglesworth / AP

David Cameron is also backing his country to stay in the EU but the issue has divided not only the British public but the prime minister's own Cabinet.

The leaders will also discuss Anglo-American efforts to combat ISIS, among other issues.

Related: Annie Leibovitz Photos Show Relaxed Queen, Little Royals

Lunch with the Queen would be the first of two royal engagements.

Later on Friday the Obamas will have dinner at Kensington Palace with perhaps the world's most popular royals, the Duke and Duchess of Cambridge — more commonly known as Prince William and Kate — and William's brother, Prince Harry.

Obama managed to make a splash before he so much as had a sip of tea with a British royal or a politicians.

Writing an op-ed in the Daily Telegraph, the president waded into the referendum on the U.K.'s possible "Brexit" from the EU, a debate that has divided Cameron's government as well as the British people.

Related: 'Brexit' Vote: Why Britain Could Quit EU and Why America Cares

"The United States sees how your powerful voice in Europe ensures that Europe takes a strong stance in the world, and keeps the EU open, outward looking, and closely linked to its allies on the other side of the Atlantic," he wrote.

"Now is a time for friends and allies to stick together," Obama wrote, adding that the decision was "a matter of deep interest to the United States."

The article drew ire from London Mayor Boris Johnson, the leading voice in the campaign to leave the EU

"It’s a breathtaking example of the principle of do-as-I-say-but-not-as-I-do," Johnson wrote in The Sun newspaper of Obama's interjection, arguing that America would never enter into an E.U.-style agreement.

"Can we take back control of our borders and our money and our system of Government? Yes we can," he added, turning Obama's 2008 campaign slogan against him.