The Queen and Duke of Edinburgh will host a private lunch for US president Barack Obama and his wife today.
Mr Obama and the First Lady will dine at Windsor Castle - the day after the Queen's 90th birthday celebrations.
The Duke and Duchess of Cambridge and Prince Harry have also invited the Obamas to have dinner with them at Kensington Palace this evening.
The US leader and his wife Michelle have developed a close bond with the Queen and her family, with the First Lady and the monarch striking up a firm friendship during the president's 2011 state visit to the UK.
When the Queen hosted the Obamas at Buckingham Palace in 2011, the First Lady was seen putting her arm around the monarch's waist, showing how the women had become firm friends.
In an ITV documentary to mark the Queen's 90th birthday, Mr Obama praised the Queen for her "generous human touch'' as he credited the monarch with helping to ensure the lasting special relationship between the UK and America.
The Obamas are likely to arrive bearing a birthday gift for the Queen who went on a celebratory walkabout in Windsor yesterday meeting some of the many well-wishers who flocked to the town centre.
They met William and Kate in May 2011 when the Cambridges carried out their first royal duty as a married couple - less than a month after their wedding.
Newly-weds William and Kate greeted the Obamas in the 1844 room of Buckingham Palace at the start of the US state visit to the UK.
William also met Mr Obama in the Oval Office in Washington DC in 2014 when the Duke told him he was so excited after the birth of Prince George that he forgot to ask whether he was a boy or a girl.
Last year, Mrs Obama took time out of her whirlwind trip to Britain to present gifts to Prince George and his newborn sister Princess Charlotte.
Mrs Obama met the young royals at a private reception at Kensington Palace, where she presented the two young children with presents.
She gave baby Charlotte an American-made Amish rocking chair and a personalised Alpaca baby blanket and George a red American-made children's wagon known as a Berlin Flyer P-Wee Wagon.
Harry has a close friendship with Mrs Obama in particular - through his support of wounded military personnel and work with the Invictus Games, a Paralympic-style championship for injured military personnel from across the globe. Mrs Obama is a keen supporter of forces' families in the US.
After the lunch Mr Obama is expected to hold bilateral talks with David Cameron in Downing Street.