The Duke of Edinburgh returned to his public duties on Thursday as he became the first member of the royal family to travel overseas this year.
Prince Edward, 60, represented the British monarchy at the funeral of former US President Jimmy Carter in Washington D.C.
The King's brother was dressed respectfully in a dark suit as he was seated alongside outgoing Canadian Prime Minister Justin Trudeau and UN Secretary-General Antonio Guterres at the state funeral at Washington National Cathedral.
Former British Prime Minister Gordon Brown was also pictured sitting behind Edward during the service, while all five living presidents gathered to pay tribute to President Carter.
President Joe Biden delivered the eulogy at the funeral, which was also attended by Bill Clinton, George W Bush, Barack Obama and President-elect Donald Trump, whose inauguration will take place on 20 January.
Jimmy Carter died aged 100 on 29 December, with his body lying in state at the US Capitol Rotunda before the funeral. The 39th US President and Nobel Prize winner will be laid to rest in his hometown Plains in Georgia.
During his time in office from 1977 to 1981, President Carter paid an official visit to the UK, where he met with the late Queen Elizabeth II and Prince Philip.
Paying tribute after his death was announced last month, the King said in a message to President Joe Biden and the American people: "It was with great sadness that I learned of the death of President Carter.
"He was a committed public servant, and devoted his life to promoting peace and human rights.
"His dedication and humility served as an inspiration to many, and I remember with great fondness his visit to the United Kingdom in 1977.
"My thoughts and prayers are with President Carter’s family and the American people at this time."
Prince Edward has not been joined by his wife, the Duchess of Edinburgh, in the US.
The couple spent Christmas on the Sandringham estate with their children, Lady Louise Windsor, 21, and James, Earl of Wessex, 17.
Edward and Sophie are both full-time working royals and will carry out a joint trip to Nepal next month.
Meanwhile, their daughter, Lady Louise, is expected to resume her studies at the University of St Andrews at the end of January.