Britain's Queen Elizabeth (C) and Prince Philip (L) present gifts to Pope Francis during a meeting at the Vatican April 3, 2014.

Britain's Prince Philip, the Duke of Edinburgh (L) meets Ian "Spike" Betterton, Chair of The Friends of St Georges Park, during a visit to Kidderminster in central England July 11, 2012. Prince Philip was accompanying Britain's Queen Elizabeth during her nationwide Diamond Jubilee tour.

Britain's Prince Philip, the Duke of Edinburgh, leads members of the royal family as they arrive to attend the Christmas Day church service on the Sandringham estate in eastern England, Britain, December 25, 2017.

Britain's Queen Elizabeth watches as Prince Philip feeds an elephant during a visit to Whipsnade Zoo where the Queen opened the new Centre for Elephant Care, in Dunstable, Britain April 11, 2017.

Britain's Queen Elizabeth and Prince Philip, the Duke of Edinburgh walk at Broadlands in Romsey, southern England in this undated photograph taken in 2007. The Queen and Prince Philip will mark their diamond wedding anniversary with a special service of thanksgiving on November 19, 2007.

Britain's Prince Philip and Queen Elizabeth leave Buckingham Palace in London, Britain, May 4, 2017.

Britain's Queen Elizabeth sits with Prince Philip as she delivers her speech in the House of Lords, during the State Opening of Parliament at the Palace of Westminster in London June 4, 2014.

Britain's Queen Elizabeth II and the Duke of Edinburgh view the restored bed-chamber at the Maritime Museum in London, in this May 1, 1990 file photo. The Queen reopened the Queen's house at the Museum which has recently been refurbished at a cost of five million pounds sterling.

Britain's Queen Elizabeth leaves with Prince Philip after a service of thanksgiving for her 90th birthday at St Paul's Cathedral in London, Britain, June 10, 2016.

His Royal Highness Prince Philip, Duke of Edinburgh entrusts new regimental colours to Lieutenant Bronson Peacock from 3rd Battalion of The Royal Canadian Regiment at a Queen's Park ceremony in Toronto April 27, 2013. The Duke of Edinburgh has been the Colonel-in Chief of the Royal Canadian Regiment since 1953.

Prince Philip, Duke of Edinburgh looks up after Britain's Queen Elizabeth (not pictured) dedicated a wreath of flowers at the site of the September 11, 2001 World Trade Center attack during her visit to New York July 6, 2010.

Paraguayan dictator Alfredo Stroessner (R) rides with Britain's Prince Philip, the Duke of Edinburgh, during a visit by the Prince in downtown Asuncion, Paraguay, in this 1963 file photo. Former Paraguayan dictator Stroessner died on August 16, 2006 at a hospital in the Brazilian capital Brasilia, where he has lived in exile since being toppled in a 1989 military coup, the hospital said.

Britain's Queen Elizabeth and Prince Philip leave after the Afghanistan service of commemoration at St Paul's Cathedral in London March 13, 2015.

Britain's Queen Elizabeth and Prince Philip wait to greet guests prior to a state banquet at Bellevue presidential palace in Berlin, Germany June 24, 2015. Queen Elizabeth and the Duke of Edinburgh are on a three day state visit to Germany.

Britain's Queen Elizabeth and Prince Philip stand on the balcony of Roemer townhall in Frankfurt, Germany June 25, 2015. Queen Elizabeth and the Duke of Edinburgh are on a three day state visit to Germany.

Prince Philip during a garden party at Hillsborough Castle, County Down in Northern Ireland, July 2001. REUTERS/Files

FILE IMAGE: Britain's Prince Philip, in his role as Captain General, Royal Marines, attends a Parade to mark the finale of the 1664 Global Challenge, on the Buckingham Palace Forecourt, in central London, Britain August 2, 2017. The 96-year-old husband of Britain's Queen Elizabeth, made his final solo appearance at the official engagement on Wednesday, before retiring from active public life.

Britain's Queen Elizabeth and Prince Philip attend Trooping the Colour in London, Britain, June 17, 2017.

An official wedding photograph released by the Duke and Duchess of Sussex, May 19, 2018. Alexi Lubomirski/via Reuters

China's President Xi Jinping and Britain's Prince Philip review an honour guard during his official welcoming ceremony in London, Britain, October 20, 2015. Xi is on a State visit to Britain.

Britain's Queen Elizabeth and Prince Philip watch the sack race at the annual Braemar Highland Gathering in Braemar, Scotland, Britain September 5, 2015. Queen Elizabeth will become Britain's longest-ever serving monarch on September 9.

Members of the royal family, including Catherine, Duchess of Cambridge holding Princess Charlotte, Prince George, Prince William, Queen Elizabeth, and Prince Philip stand on the balcony of Buckingham Palace after the annual Trooping the Colour ceremony on Horseguards Parade in central London, Britain June 11, 2016. Trooping the Colour is a ceremony to honour Queen Elizabeth's official birthday. The Queen celebrates her 90th birthday this year.

Britain's Queen Elizabeth and her husband Prince Philip leave the Houses of Parliament following the annual State Opening of Parliament in London, November 18, 2009.

Britain's Prince Philip, Duke of Edinburgh attends the Epsom Derby in Epsom, south of London, June 1, 2013.

FILE IMAGE: Britain's Prince Philip waits for the bridal procession following the wedding of Princess Eugenie of York and Jack Brooksbank in St George's Chapel, Windsor Castle, near London, Britain October 12, 2018. Alastair Grant/Pool via REUTERS

Queen Elizabeth II and Prince Phillip, wearing their Order of the Garter robes, ride in an open-topped carriage to Windsor Castle following the Garter Ceremony, June 18, 2001. REUTERS/File

Britain's Queen Elizabeth and Prince Philip walk through a field of ceramic poppies that form part of the art installation "Blood Swept Lands and Seas of Red", at the Tower of London in London October 16, 2014. The evolving art installation, which will be completed on November 11, will create a commemoration for the centenary of World War One.

Britain's Queen Elizabeth and Prince Philip are driven past well-wishers during celebrations for the Queen's 90th birthday, in Windsor, Britain April 21, 2016.