Pippa, 37, and James, 45, are already parents to two-year-old Arthur Michael William, who was born on 15 October 2018 in the Lindo Wing at St Mary's Hospital in London.
Page Six reports that the couple are expecting a baby in 2021, citing a source close to the family.
WATCH: Meet the Middletons: everything you need to know about Kate's family
The new baby will be the fifth grandchild for Pippa's parents Carole and Michael Middleton, who are already grandparents to Pippa's son Arthur, and Prince William and Kate's three children, Prince George, Princess Charlotte and Prince Louis.
Pippa also had her own major role as Kate's maid of honour at the royal wedding in April 2011.
Pippa and James married in 2017
Former professional racing driver James is the son of David Matthews (who holds the title of Laird of Glen Affric), and artist Jane Matthews. The couple own the five-star Eden Rock hotel in St. Barts, where the Middletons holidayed at Christmas 2019. James's younger brother is former Made in Chelsea star Spencer Matthews.
Pippa confirmed she was expecting her first child Arthur in her fitness column for Waitrose Weekend magazine in June 2018, where she spoke about exercising during pregnancy.
The author, who shares a love of sport with sister Kate, wrote: "I've noticed my body change but through exercise I feel it's being strengthened."
Pippa during her first pregnancy with son Arthur
James and Pippa live in a £17million mansion in west London and Kate's sister is often spotted strolling through the capital with Arthur.
Before she became the Duchess of Cambridge, Kate used to share an apartment with Pippa on Old Church Street in Chelsea, London.
Carole and Michael bought the property for their daughters for £780,000 back in 2002 and put it up for sale for £1.95million in 2018, but it sold for £1.88million in September 2019.
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