Recalling William’s own birth three decades ago, Spencer, 49, said he was at boarding school at Eton when a tutor alerted him.
“The teacher rushed into my bedroom to tell me the good news, a bottle of champagne in his hands,” Spencer tells PEOPLE in a statement.
“Now, 31 years on, my wife and I are with all nine of our children, and we’re celebrating this family addition with just as much happiness.”
This marks the first statement from a member of the maternal side of William’s family.
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• See How the World Welcomed the Royal Baby
• What Will Kate & William Name Their Baby?
• Will & Kate’s Road to Parenthood