Britain's Queen Elizabeth (R) and Prince Philip (L) talk as they watch the caber being tossed at the annual Braemar Highland Gathering in Braemar, Scotland September 6, 2014. The referendum on Scottish independence will take place on September 18, when Scotland will vote whether or not to end the 307-year-old union with the rest of the United Kingdom.

Britain's Prince William, Duke of Cambridge, Catherine, Duchess of Cambridge, Prince Philip, Duke of Edinburgh and Queen Elizabeth II arrive to greet guests attending a garden party at Buckingham Palace on May 24, 2016 in London, Britain.

Britain's Queen Elizabeth and Prince Philip leave after the State Opening of Parliament at the Palace of Westminster in central London, Britain May 27, 2015.

Horse Racing - Investec Derby Festival - Epsom Racecourse - 6/6/15 Britain's Queen Elizabeth and her husband Prince Philip in the stands Reuters / Peter Nicholls

Britain's Queen Elizabeth and her husband Prince Philip watch performers during a visit to the Manchester Central Convention Centre in Manchester, northern England March 23, 2012. The Queen is travelling around Britain to celebrate her Diamond Jubilee.

Queen Elizabeth II and Prince Philip pose with U.S. President John F. Kennedy and First Lady Jacqueline Kennedy at Buckingham Palace in London, Britain, June 5, 1961. Picture taken June 5, 1961. U. S. Department of State/John F. Kennedy Presidential Library and Museum/Handout via REUTERS.

Prince Charles (R) with his sons Prince William (2nd L) and Prince Harry (2nd R), his father the Duke of Edinburgh (L) and Earl Spencer the brother of Diana, Princess of Wales walk forward to join the funeral cortege bearing the coffin of the Princess to Westminster Abbey, September 6. Millions of mourners lined the route from Kensington Palace to the Abbey.

Britain's Queen Elizabeth and Prince Philip arrive by boat at the Floriade flower festival in Canberra October 20, 2011. Queen Elizabeth is in Australia for an eleven-day official visit.

Queen Elizabeth II and Prince Philip attend the Beating the Retreat ceremony on Horseguards Parade, June 13. The Retreat, which was played by the massed bands of the Royal Marines, was in honour of Prince Philip's 75th birthday, Philip is the Captain General of the Royal Marines.

The Duchess of Cornwall, the Prince of Wales, Queen Elizabeth II, the Duke of Edinburgh, and Duke and Duchess of Cambridge (L-R) arrive for the annual evening reception for members of the Diplomatic Corps at Buckingham Palace in London, Britain December 8, 2016.

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 Prince Philip leaves after a service of thanksgiving for Queen Elizabeth's 90th birthday at St Paul's Cathedral in London, Britain, June 10, 2016. REUTERS/Ben Stansall/Pool - RTSGWC2

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.

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.

The hand of Britain's Queen Elizabeth II (L) is held by her husband, Prince Philip, Duke of Edinburgh, after her speech at the State Opening of Parliament in the House of Lords in London, November 13, 2002. Plans for a crackdown on crime and anti-social behaviour were at the heart of the Queen's Speech on November 13, which also included controversial plans for hospitals, jury trials and licensing laws.

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 and her husband Prince Philip pause during a tour of the Royal London Hospital in east London on February 27, 2013. The Queen opened the revamped hospital on Wednesday.

(Front L-R) Britain's Queen Elizabeth, Prince Philip, Prince Harry, Prince William, the Duke of Cambridge and his wife, Catherine, the Duchess of Cambridge share a light moment as they stand on the balcony of Buckingham Palace in the annual Trooping of the Colour ceremony to celebrate the Queen's official birthday in central London, June 14, 2014. REUTERS/Toby Melville

Britain's Queen Elizabeth (R) and Prince Philip, The Duke of Edinburgh, view horses in the parade ring at the Epsom Derby festival, part of the Queen's Diamond Jubilee celebrations, in Epsom, southwest of London June 2, 2012. Queen Elizabeth gets four days of celebrations to mark her 60 years on the British throne under way on Saturday with one of her favourite pastimes, a trip to the horse races, as tributes to the long-serving monarch pour in. REUTERS/Toby Melville

Britain's Queen Elizabeth presents her husband Prince Philip with New Zealand's highest honour, the Order of New Zealand at Buckingham Palace in London June 6, 2013. Prince Philip celebrates his 92nd birthday on June 10.

Britain's Queen Elizabeth speaks to her husband Prince Philip as they attend the official re-opening of the Cutty Sark in Greenwich, London April 25, 2012. The Cutty Sark is the world's last surviving tea clipper and has been extensively restored after being damaged by fire in 2006. REUTERS/Paul Hackett

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.

Britain's Queen Elizabeth receives the trophy from Prince Philip, Duke of Edinburgh, after her horse Estimate won The Queen's Vase on the fourth day at Royal Ascot, southwest of London June 22, 2012. REUTERS/Stefan Wermuth

Britain's Queen Elizabeth and Prince Philip laugh after bidding farewell to the President of Ireland Michael D. Higgins and his wife Sabina at Windsor Castle in Windsor, southern England April 11, 2014. The Irish President and his wife Sabina left Windsor at the end of a four day State Visit to Britain, during which they stayed at Windsor Castle as guests of Queen Elizabeth.

A pigeon flies ahead of Britain's Queen Elizabeth and Prince Philip, and Princes William, Harry and Kate Duchess of Cambridge behind as they wave to guests attending the Patron's Lunch on the Mall, an event to mark her 90th birthday, in London, June 12, 2016.

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.

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.

Britain's Queen Elizabeth travels with Prince Philip on the new Scottish Borders railway line, in Scotland, September 9, 2015. Queen Elizabeth officialy opened the new Scottish Borders Railway on the day she became Britain's longest reigning monarch.

Britain's Queen Elizabeth and Prince Philip arrive for the fourth day of racing at Royal Ascot in southern England June 18, 2010. REUTERS/Stefan Wermuth

Britain's Queen Elizabeth kisses her husband the Duke of Edinburgh at midnight in the Millennium Dome January 1. The Queen and Prince Philip joined Prime Minister Tony Blair and 10,000 party goers to celebrate the new millennium at the Dome on the Greenwich Meridian.

Britain's Queen Elizabeth and Prince Philip take a boat trip on Spree river in Berlin, Germany June 24, 2015.

Britain's Queen Elizabeth (3L) stands with her husband Prince Philip (R), son Prince Charles (2R) and grandsons princes William (L) and Harry (2L) during the unveiling of a memorial fountain dedicated to the late Princess Diana at Hyde Park in London, July 6, 2004.

Britain's Queen Elizabeth II (R) and her husband Prince Phillip (L) watch as an orphan performs at the Mildmay HIV Centre in Kampala, Uganda November 22, 2007, on the first day of her state visit to Uganda. The Queen is due to officiate at the opening of the Commonwealth Heads of Government Meeting (CHOGM) November 22, 2007.

Prince Philip talks with one of the riders after horses from a local ranch were ridden down to the station to form a Guard of Honor during the visit of Queen Elizabeth II and Prince Philip to Kamloops, British Columbia, Canada in July 1959. Picture taken in July 1959. Gar Lunney/Library and Archives Canada/National Film Board of Canada.

Britain's Prince William and his bride Catherine, Duchess of Cambridge (C), pose for an official photograph, with their families, on the day of their wedding, in the throne room at Buckingham Palace, in central London April 29, 2011.