The former US President celebrated his 60th birthday with an extravagant shindig on Saturday. The festivities were attended by around 200 family members and close friends, which included many Hollywood stars. Among those partying with the Obamas at their 29-acre Martha's Vineyard estate were Tom Hanks and his wife Rita Wilson, as well as Steven Spielberg.

       However, conspicuous by their absence were the Duke and Duchess of Sussex, which has led to all sorts of speculation as to the reasons why.

       Camilla Tominey, a Royal Family expert and associate editor at The Daily Telegraph, wrote: "It was thought that the newly California-based couple would be shoo-ins at Obama’s 60th, as prominent ‘progressives’ and newfound members of the US metropolitan liberal elite."

       She suggested that the former US President and his wife may not have been impressed by the manner in which the Duke and Duchess spoke of Royal Family members in their bombshell interview with TV host Oprah Winfrey.

       Ms Tominey continued in The Telegraph piece: "So it arguably will not have gone down particularly well with a couple that have always put 'family first' to see Harry and Meghan being so openly critical of their Royal relatives during their Oprah interview in March."

       Barack and Michelle are said to enjoy a warm relationship with the Queen and have enjoyed her company on three separate occasions.

       They first met Her Majesty on a 2009 state visit to the UK, attending a banquet in their honour at Buckingham Place.

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       The US First couple then returned in 2011, when they were introduced to Prince William and Kate Middleton.

       Their final meeting with the British sovereign was in April 2016, several months before Donald Trump was elected the new US President.

       A former employee told royal author Tom Quinn that Her Majesty "completely fell in love" with Mr Obama and would like to invite him again to Buckingham Palace, even though he is no longer President.

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       "The Queen has a soft spot for Americans after meeting Barack Obama who she completely fell in love with – so much so that she has frequently asked her courtiers if they could arrange for him to come to Britain now he is no longer president," they said.

       The Queen appeared to form a close relationship with Michelle Obama, with the two women memorably photographed with their arms around one another during the 2009 visit.

       According to Angela Kelly, the Queen's dressmaker, the "two remarkable women" shared an "instant and mutual warmth."