The Queen is to become a great-grandmother for a second time.
Peter Phillips, the Queen's eldest grandson, and his Canadian wife Autumn are expecting their second child in March, the BBC's royal correspondent Peter Hunt said.
Mr Phillips' mother, the Princess Royal, will become a grandmother for the second time.
A spokesman for Buckingham Palace said the Queen and the Duke of Edinburgh were "delighted" at the news.
Mr and Mrs Phillips already have a daughter, Savannah, who is 12th in line to the throne.
The couple attracted media attention when they sold their wedding photos to Hello! magazine in 2008.
At the time Liberal Democrat MP Norman Lamb called the move "a serious error of judgment".
Earlier this year Mr Phillips' sister Zara married England rugby player Mike Tindall.