Sarah was the only sister of Zara's father, Captain Mark Phillips, who was married to Princess Anne from 1973 until 1992. "She died suddenly," a friend of the family told the Daily Mail. "It is very sad."
Zara Phillips with her father, Captain Mark Phillips
A funeral is expected to take place on Friday at the Church of St Peter and St Paul in Great Somerford, Wiltshire.
Sarah was always very discreet about her royal connections. "It's not really done to talk about the children, I'm afraid," she once said of Zara and her brother Peter Phillips. "You see the Princess Royal and their father don't really like the idea of them being written about in newspapers."
Her death will come as a big blow to Zara, who is preparing for her first Christmas with little Mia.
Zara Phillips and Mike Tindall are looking forward to their first Christmas with baby Mia
Zara and her husband Mike Tindall live at Gatcombe Park in Gloucestershire, the home of Princess Anne. But they are expected to join the rest of the royal family for Christmas, including a Christmas Eve dinner at the Queen’s Sandringham House.
The royals will then attend a Christmas Day church service followed by a big lunch together.