Rare Party: President Obama, first lady Michelle and Queen Elizabeth, Prince William, and Kate Middleton [PHOTOS]
President Barack Obama (L) and first lady Michelle Obama (R) talk to Prince William (2nd L) and Catherine, Duchess of Cambridge at Buckingham Palace, in London May 24, 2011. REUTERS

The royals are hosting US President Obama and First Lady Michelle to a private dinner on Friday 22 April at Kensington Palace. "The Duke and Duchess of Cambridge as well as Prince Harry are very much looking forward to welcoming President and Mrs Obama to Kensington Palace," a statement from Kensington Palace read.

The President of the United States was first introduced to Prince William and the Duchess of Cambridge in May 2011 during the couple's first royal duty as a married couple. In 2014, Prince William met Obama again while on a visit to the White House. On this US tour, the duchess was pregnant with Princess Charlotte and stayed in New York, visiting to a child-development centre in Harlem.

The four were last photographed together in 2011, when President Obama and First Lady Michelle called by Kensington Palace during a one-day trip to London. The US first lady has previously praised the duchess as "a passionate voice on so many important issues, and I'm grateful that she is using her day as guest editor [at the Huffington Post] to shine a bright light on mental health, particularly children's mental health, and on the tens of millions of people who suffer in silence," according to BBC News.

The Obamas are arriving in Britain later this week to join the Queen at Windsor Castle to celebrate her 90th birthday. On Friday they will also have lunch with Queen Elizabeth and Prince Philip at Windsor Castle, before going on to their dinner with the younger royals - William, Kate and Harry.

A White House spokesman said: "The visit will allow the president to offer his gratitude to the British government and people for their stalwart partnership with his administration and the American people throughout his presidency."

The visit is believed to be Obama's last official trip to Britain as the United States president. During his trip, the US president will hold a joint conference with British PM David Cameron at 10 Downing Street.