In November 2010, William famously gave Kate the engagement ring that once belonged to Diana, saying it was "my way of making sure my mother didn't miss out on today and the excitement".
Following Mr Morton's claims, Elizabeth became 10/1 favourite for the name with Coral. James is second at 11/1, Victoria third at 12/1 and Alexandra fourth at 14/1.
Kate's latest pregnancy was announced on September 8, as it was revealed she was suffering from very acute morning sickness and being treated by doctors at Kensington Palace.
The couple are choosing to base their family in Norfolk, but the new baby will be delivered at St Mary's Hospital in Paddington, central London, where Prince George was born in July 2013.
Mr Morton, 60, wrote international bestseller Diana: Her True Story in 1992 and a revised edition after her death based on taped responses from Diana to his written questions.
Those recordings, about seven hours of taped responses, were secretly made in Kensington Palace in 1991 during a series of interviews conducted through an intermediary.
In the tapes, the princess talks openly about her life before marrying Prince Charles, her long battle with bulimia, her suicide attempts, the heartache over her husband's long affair with Camilla Parker Bowles and her strained relationships with other members of the Royal family.
- Daily Mail