The moniker was revealed by The Mail on Sunday reports, citing the baby’s birth certificate.
The 35-year-old most likely chose Michael in honour of James’ brother Michael Matthews, who died at age 22 as he climbed Mount Everest in 1999. Michael is also the name of Pippa’s father.
While the baby's first name - Arthur - is also Prince George and Prince Louis' middle name.
Kate Middleton’s younger sister welcomed Arthur Michael William on October 15th. Just like his royal cousins, he was born at the private Lindo Wing of St. Mary’s Hospital in London. His birth came just as the world was learning that Prince Harry and Meghan Markle are expecting a baby of their own.
Arthur was born weighing 8lb 9oz, a spokeswoman for Pippa and James Matthews said, adding that "everyone is delighted and mother and baby are doing well."