A Kensington Palace spokesman said, "The Duke and Duchess of Cambridge are absolutely delighted with the news."
An excited family friend tells WHO, "Woo hoo! She kept that under the radar! Good move!"
Middleton and Matthews first dated in 2012 before subsequently splitting. The fitness fanatic then dated Nico Jackson for three years until they split in 2015.
Three months later, the couple re-lit their flame when, Matthews, 40, was spotted by Pippa's side during a St. Barts vacation with her younger brother, James Middleton.
"I saw them a couple of months walking around the village – just casually strolling through the woods having a chat. They seemed very happy together," says the family friend, adding, ""It's wonderful news. I've met James and a he's a really nice guy."
Matthews, who runs Eden Rock Capital Management in London's swanky Mayfair district, also vacationed with Pippa and her parents Michael and Carole Middleton in August, at his parents' hotel, Eden Rock, in St. Barts.
