Zara Phillips and Mike Tindall married at Canongate Kirk, in Edinburgh, in July 2011.
The birth will follow the arrival of Kate Middleton and Prince William’s baby, which is due this month.
In a statement, Buckingham Palace said: “Mr. and Mrs. Mike Tindall are very pleased to announce that Zara Tindall is expecting a baby in the new year.”
It went on: “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.”
British PM David Cameron’s official spokesman described the announcement as “quite terrific news”.
He told reporters: “The prime minister will send his very, very best wishes to the happy couple.”
The new baby will become 16th in line to the throne and the fourth great grandchild to the Queen and the Duke of Edinburgh.
The others are Savannah and Isla Phillips, daughters of Zara’s brother, Peter, and the Duchess of Cambridge’s expected baby.
Zara Phillips is currently 14th in line to the throne, but does not hold a royal title and is not an HRH.
Her children will be titled Miss or Master Tindall.
The couple met during England’s Rugby World Cup-winning campaign in Australia in 2003.
Former England rugby star Mike Tindall is now a player-coach at Gloucester.
Miss Zara Phillips has achieved sporting success in her own right, winning individual gold in three-day eventing at the World Equestrian Games in Germany in 2006.
As a member of the Great Britain eventing team, Zara Phillips won a silver medal at the London 2012 Olympics.
New line of the British royal succession:
1. Prince Charles
2. Prince William
3. “Baby Wales” (Kate Middleton and Prince William’s first child)
4. Prince Harry
5. Prince Andrew
6. Princess Beatrice
7. Princess Eugenie
8. Prince Edward
9. Viscount Severn (James -Edward’s son)
10. Lady Louise (Edward’s daughter)
11. Princess Anne
12. Peter Phillips
13 & 14. Savannah and Isla Phillips (Peter’s daughters)
15. Zara Tindall (nee Phillips)
16. “Baby Tindall”
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