The actress had signed up to star in new comedy show Tom and Jenny, about a warring young couple forced to live together after a break-up.
However, Piper has bowed out of the cast after giving birth to her second son Eugene in April (12). She also has a three-year-old son, Winston James, with husband Laurence Fox.
Her decision comes just weeks after the Secret Diary of a Call Girl star hinted at a move to the U.S. to boost her career.
She said, “It’s desperate. You slightly lose your mind. There’s not much work in the U.K. – that’s why everyone’s moving to America. It’s where the work seems to be.”