The Queen's granddaughter Zara Tindall and her husband Mike Tindall are expecting their second child, a spokeswoman for the couple has said.
The Queen and the Royal Family were "very pleased" to hear the news, a Buckingham Palace spokesman said.
The couple, who have a three-year-old daughter, Mia, have previously lost a baby during pregnancy.
The baby is believed to be due in the summer and would be the Queen's seventh great grandchild.
'Welcoming new members'
The Duke and Duchess of Cambridge expect to welcome their third child in April, which will be the Queen's sixth great grandchild.
In her Christmas speech, the Queen said her family "look forward to welcoming new members into it next year".
Alongside the impending arrival of two great grandchildren, Prince Harry is due to marry Meghan Markle in May.
The baby announcement was made as former equestrian champion Mrs Tindall and former England rugby player Mr Tindall enjoyed a working holiday in Australia.
Mrs Tindall, who is the daughter of the Queen's only daughter, the Princess Royal, married Mr Tindall in 2011.
As a member of the Great Britain eventing team, she won the Eventing World Championship in 2006 and was named BBC Sports Personality later that year, before securing a silver medal at the London 2012 Olympics.
Mr Tindall, 35 - who won the 2003 Rugby World Cup with England - does not hold a royal title and is not an HRH.
If as expected Mr and Mrs Tindall's baby is born after the Duke and Duchess of Cambridge's, the child will be the 19th in line to the throne.
The couple had revealed in November 2016 they were expecting a baby in late spring 2017 but the following month announced they had lost the baby.