Queen Elizabeth and Prince Philip walk at Broadlands in Romsey, southern England in 2007.
REUTERS/Fiona Hanson/Pool 

Queen Elizabeth and Prince Philip talk with Margot Fonteyn at a CG Gala in June 1958. REUTERS/Stringer 

Prince Philip rides with Paraguayan dictator Alfredo Stroessner in downtown Asuncion in 1963. REUTERS/R. E. Adorno 

Paraguayan dictator Alfredo Stroessner decorates Prince Philip during a visit by the Prince to Paraguay, in 1963.
REUTERS/R. E. Adorno 

Queen Elizabeth and Prince Philip view the restored bed-chamber at the Maritime Museum in London, May 1990. REUTERS/Mike Parsson 

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

Prince Philip looks up after Queen Elizabeth dedicated a wreath of flowers at the site of the September 11, 2001 World Trade Center attack during her visit to New York, July 2010. REUTERS/Lucas Jackson 

Prince Philip attends the Epsom Derby in Epsom, south of London, June 1, 2013. REUTERS/Andrew Winning 

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 

Queen Elizabeth and her husband Prince Philip leave the Houses of Parliament following the annual State Opening of Parliament, November 18, 2009. REUTERS/Toby Melville 

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. REUTERS/Hannah McKay TPX IMAGES OF THE DAY 

Britain's Prince Philip and Queen Elizabeth leave Buckingham Palace in London, Britain, May 4, 2017. REUTERS/Neil Hall TPX IMAGES OF THE DAY 

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. REUTERS/Hannah McKay 

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. REUTERS/Toby Melville

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. REUTERS/Peter Nicholls TPX IMAGES OF THE DAY 

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. REUTERS/Toby Melville TPX IMAGES OF THE DAY 

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. REUTERS/Suzanne Plunkett TPX IMAGES OF THE DAY 

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. REUTERS/Russell Cheyne TPX IMAGES OF THE DAY

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. REUTERS/Boris Roessler/Pool TPX IMAGES OF THE DAY 

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. REUTERS/Wolfgang Kumm/Pool TPX IMAGES OF THE DAY

Britain's Queen Elizabeth and Prince Philip leave after the Afghanistan service of commemoration at St Paul's Cathedral in London March 13, 2015. REUTERS/Stefan Wermuth (BRITAIN - Tags: ROYALS MILITARY RELIGION POLITICS ANNIVERSARY CONFLICT TPX IMAGES OF THE DAY)

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. REUTERS/Luke MacGregor (BRITAIN - Tags: ENTERTAINMENT ROYALS SOCIETY TPX IMAGES OF THE DAY) 

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. REUTERS/Suzanne Plunkett (BRITAIN - Tags: ENTERTAINMENT POLITICS SOCIETY ROYALS TPX IMAGES OF THE DAY) 

Britain's Queen Elizabeth (C) and Prince Philip (L) present gifts to Pope Francis during a meeting at the Vatican April 3, 2014. REUTERS/Stefano Rellandini (VATICAN - Tags: RELIGION ROYALS TPX IMAGES OF THE DAY) 

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. REUTERS/Fred Thornhill (CANADA - Tags: ENTERTAINMENT MILITARY POLITICS ROYALS TPX IMAGES OF THE DAY) 

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. REUTERS/Heathcliffe O'Malley/Pool (BRITAIN - Tags: SOCIETY ROYALS TPX IMAGES OF THE DAY) 

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 

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

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. REUTERS/Hannah McKay TPX IMAGES OF THE DAY