The Duke of Edinburgh began teaching carriage-driving to Countess Mountbatten of Burma almost 30 years ago.
The 67-year-old countess, Penelope “Penny” Knatchbull, previously known as Lady Romsey and later Lady Brabourne, became one of Philip’s closest confidantes.
Philip first began teaching her the daredevil equestrian sport in 1994 and they remained firm friends for the rest of his life.
They were often pictured together at the Royal Windsor Horse Show, sometimes riding mini motorcycles around the carriage driving course.
Countess Mountbatten, who will join mourners at Philip’s funeral, is the wife of Earl Mountbatten, Norton Knatchbull, the grandson of Philip’s beloved uncle the 1st Earl Mountbatten, who was murdered by the IRA in 1979.
She was a regular visitor at the duke’s Wood Farm cottage at Sandringham after his retirement.
The countess is understood to have formed a close friendship with the Queen and Philip after her daughter Leonora died aged five from cancer in 1991.
Such was her bond with the royal couple that palace staff were said to have nicknamed her “and also”, because she was automatically added to guest lists.