Zara Phillips and Mike Tindall have announced they are expecting their first baby in the new year.
The birth will follow the keenly anticipated arrival of the Duke and Duchess of Cambridge's baby, which is expected in the coming days.
Buckingham Palace, using Zara's married name, said in short statement: "Mr and Mrs Mike Tindall are very pleased to announce that Zara Tindall is expecting a baby in the new year. The Princess Royal and Captain Mark Phillips, Mr Phillip and Mrs Linda Tindall, and members of both families are delighted with the news. This baby will be the first child for Mike and Zara and the third grandchild for the Princess Royal."
David Cameron's official spokesman described the announcement as "quite terrific news", telling reporters: "The Prime Minister will send his very, very best wishes to the happy couple."
Zara, a silver medal-winning Olympian and the Queen's granddaughter, is one of Britain's leading equestrian riders.
The royal took a tumble while competing at the Barbury International Eventing and Show in Marlborough, Wiltshire, a few days ago. But she was described as "fine" by a spokeswoman for Juniper Sport, a PR and sports agency that represents Ms Phillips.
The spokeswoman added that Zara was one of the many famous faces at Wimbledon on Sunday to see Andy Murray make history by winning the men's title.
The royal won a London 2012 silver with eventing equestrian teammates William Fox-Pitt, Mary King, Tina Cook and Nicola Wilson, and is planning to take part in the Rio Games in 2016. Asked if Zara still had the next Olympics on her schedule now she was a prospective mother the spokeswoman said: "She can still compete, it totally depends on horses and whether she feels it's appropriate."
Zara married former England rugby star Mike Tindall in July 2011 in a glittering ceremony staged in Edinburgh and attended by the Queen and senior royals.
When born, her baby will be the Queen's fourth great grandchild, after her brother Peter Phillips' two children and William and Kate's infant, expected in mid July.