Her Majesty was “delighted” to hear her sixth great-grandchild is due.
Olympic equestrian champ Zara already has a two-year-old daughter called Mia.
The sporting couple wed in 2011 and the child will be second cousin to Kate and Wills’ tots Prince George and Princess Charlotte.
Mike thanked fans on Twitter, saying the couple are “very very happy about the little one on its way. 2017 is already starting well!!”
The couple live on the Princess Royal’s Gatcombe Park estate in Gloucestershire.
The baby will be 18th in line to the throne.